Optical characteristic measuring apparatus

ABSTRACT

An optical characteristic measuring apparatus includes: a light source section which sweeps wavelengths of a first input light and a second input light respectively, frequencies of the first and second input lights being different from each other and polarized states of the first and second input lights being perpendicular to each other, and outputs the first and second input light; an interference section which inputs one branched light of the first and second input lights to a measuring object, makes output light from the measuring object interfere with other branched light of the first and second input lights, and outputs a plurality of interference lights; a plurality of light receiving sections which are respectively provided for the interference lights, receives the interference lights respectively, and outputs signals in accordance with optical powers of the interference lights respectively; and a low-pass filter for filtering the outputted signals.

This is a Divisional Application of Ser. No. 11/443,344. Thisapplication claims foreign priorities based on Japanese Patentapplication No. 2005-159061, filed May 31, 2005, Japanese Patentapplication No. 2005-163825, filed Jun. 3, 2005, and Japanese Patentapplication No. 2005-171310, filed Jun. 10, 2005, the contents of whichare incorporated herein by reference in their entireties.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an optical characteristic measuringapparatus for obtaining an optical characteristic of a measuring object,particularly a transfer function matrix (for example, Jones matrix) of ameasuring object, in details, relates to an optical characteristicmeasuring apparatus capable of accurate measurement even when afrequency difference of a first and second incident lights is varied.

The present invention relates to an optical characteristic measuringapparatus having an interference section for multiplexing a first inputlight and a second input light, frequencies of which differ from eachother and polarized states of which are perpendicular to each other,inputting a multiplexed light to a measuring object, and making outputlight outputted from the measuring object interfere with at least one ofthe first input light and the second input light. The opticalcharacteristic measuring apparatus obtains an optical characteristic ofthe measuring object, particularly, a transfer function matrix (forexample, Jones matrix) of the measuring object by interference lightfrom the interference section. In details, the present invention relatesto an optical characteristic measuring apparatus capable of accuratemeasurement even when a frequency sweep speed of a waveform variablelight source is not constant.

The present invention relates to an optical characteristic measuringapparatus for measuring an optical characteristic of a measuring object,particularly a transfer function matrix (for example, Jones matrix) ofthe measuring object, by branching light from a light source section,making one branched light incident on the measuring object, and makingoutput light (signal light) outputted from the measuring objectinterfere with other branched light (reference light). In details, thepresent invention relates to an optical characteristic measuringapparatus capable of easily determining an increase or a decrease of aphase difference of light (signal light and reference light) to bemultiplexed.

2. Description of the Related Art

An optical characteristic measuring apparatus obtains opticalcharacteristics (for example, insertion loss, reflectance,transmittance, polarized light dependency, wavelength dispersion,polarization mode dispersion, and the like) of a measuring object (forexample, optical element, optical apparatus, test apparatus/measuringapparatus of the optical element or the optical apparatus or the like),specifically obtains a transfer function matrix (for example, Jonesmatrix) of a measuring object by measurement, and obtains the opticalcharacteristics of the measuring object all together, or only thenecessary optical characteristic from the transfer function.

In order to obtain the transfer function matrix by measurement, signallight having a frequency fs is made to be incident on the measuringobject, and signal light (transmitted light or reflected light)outputted from the measuring object is multiplexed with reference light(frequency fr) to interfere with each other. Further, an interferencesignal is received by a light receiving section and an amplitude and aphase of the interference signal are measured (so-to-speak heterodynedetection). Further, in order to obtain a transfer function in apredetermined measuring wavelength range, a light source is subjected towavelength sweep (frequency sweep) (refer to, for example,JP-A-2002-243585, U.S. Pat. No. 6,376,830, and JP-A-2004-20567).

FIG. 14 is a diagram showing an input/output characteristic to and froma measuring object 1. In FIG. 14, input light, output light to and fromthe measuring object 1 are represented by a column vector of 2 columnsand 1 row (so-to-speak Jones vector) representing amplitudes and phasesof two polarized light perpendicular to each other, and a transferfunction matrix (so-to-speak Jones matrix) of the measuring object 1 isshown by Equation (1) as follows.

$\begin{matrix}\left\lbrack {{Equation}\mspace{14mu} 1} \right\rbrack & \; \\\begin{pmatrix}T_{11} & T_{12} \\T_{21} & T_{22}\end{pmatrix} & (1)\end{matrix}$

In order to obtain such Jones matrix, first, second input light havingpolarized light (linearly polarized light, elliptically polarized light,circularly polarized light) polarized states of which are perpendicularto each other are inputted to the measuring object 1. Further, theamplitudes and phases of Jones vector of input light and output lightoutputted from the measuring object 1 are measured to obtain.

In order to easily obtain Jones matrix by operation from a result ofmeasuring input light, output light, generally, linearly polarized light(for example, s polarized light, p polarized light) polarization planesof which are perpendicular to each other are used for the first, thesecond input light. Further, polarized states of respective input lightof s polarized light, p polarized light to the measuring object 1 arechanged by an optical characteristic of the measuring object 1 andemitted. Further, in order to facilitate the operation, in the outputlight from the measuring object 1, linearly polarized light (forexample, s polarized light, p polarized light) polarization planes ofwhich are perpendicular to each other are interfered with referencelight to be measured.

That is, there are present emitted s polarized light and emitted ppolarized light with regard to incident s polarized light, and there arepresent emitted s polarized light and emitted p polarized light withregard to incident p polarized light. Further, the incident s polarizedlight is s polarized light inputted to the measuring object 1, and theemitted s polarized light is s polarized light outputted from themeasuring object 1. Also incident p polarized light, emitted p polarizedlight are similarly p polarized light inputted and outputted to and fromthe measuring object 1.

Therefore, in Equation (1), mentioned above, notation T₁₁ represents arelationship of emitted s polarized light relative to incident spolarized light, notation T₂₁ represents a relationship of emittedpolarized light relative to the incident s polarized light, notation T₁₂represents a relationship of emitted s polarized light relative toincident p polarized light, notation T₂₂ represents a relationship ofemitted p polarized light relative to incident p polarized light. Thatis, in notation T_(x,y) represents a polarized state of an emitting side(x=1 represents s polarized light, x=2 represents p polarized light), yrepresents a polarized state of the incident side (y=1 represents spolarized light, y=2 represents p polarized light).

For example, when input light (that is, signal light) to the measuringobject 1 is s polarized light, output light from the measuring object 1becomes light multiplexed with T₁₁ and T₂₁ and becomes light multiplexedwith T₁₂ and T₂₂ when input light is p polarized light.

In this way, it is necessary to measures polarized light, p polarizedlight having different polarization planes as input light and therefore,in a measuring method, there are a case in which measurement is carriedout by subjecting input light to wavelength sweep by s polarized lightand thereafter subjecting input light to wavelength sweep by p polarizedlight again, and a case in which measurement is carried out by one timewavelength sweep by simultaneously inputting s polarized light and ppolarized light to the measuring object 1. When measured by one timewavelength sweep, a measuring time period can be shortened andmeasurement can be carried out accurately without an error derived fromreproducibility (for example, wavelength reproducibility) in a firsttime and a second time of wavelength sweep.

However, since s polarized light and p polarized light aresimultaneously inputted to the measuring object 1, it is necessary toseparate an interference signal of s polarized light and reference lightand an interference signal of p polarized light and reference light. Inthe separation, there are a method of separating by a time region bymaking the interference signal of s polarized light and the interferencesignal of p polarized light respectively constitute different measuringoptical path difference (refer to, for example, U.S. Pat. No.6,376,830), and a method of subjecting the interference signal of spolarized light and interference signal of p polarized light tointensity modulation respectively by different frequencies to separatefrom a difference in modulated frequencies for intensity modulation(refer to, for example, JP-A-2004-20567).

However, it is very difficult to separate the interference signal basedon s polarized light and the interference signal based on p polarizedlight by the time region, when separated by the difference in themodulated frequencies, there poses a problem that a measured wavelengthrange is limited by wavelength dependency of an intensity modulator perse and the intensity modulator is very expensive.

FIRST RELATED ART

FIG. 15 is a diagram showing a configuration of an opticalcharacteristic measuring apparatus of a related art (refer to, forexample, JP-A-2002-243585). In FIG. 15, a wavelength variable lightsource 2 outputs laser light while carrying out wavelength sweep by apredetermined wavelength sweep speed. A half mirror (hereinafter,abbreviated as HM) 3 branches laser light from the wavelength variablelight source 2 in two. A polarization beam splitter (hereinafter,abbreviated as PBS) 4 branches laser light in two of light (p polarizedlight, s polarized light) polarization planes of which are perpendicularto each other. Here, p polarized light is transmitted by an optical pathOP1, and s polarized light is transmitted by an optical path OP2.

PBS 5 synthesizes light branched by PBS 4 and transmitted by thedifferent optical paths OP1, OP2 to output to the measuring object 1.Here, light inputted to the measuring object 1 is signal light. A delayfiber 6 is provided on the optical path OP2 between PBS 4, 5 and delaysone branched light.

Therefore, since incident s polarized light passes through the delayfiber 6, when a frequency of incident p polarized light is designated bynotation f1 (t), a frequency of incident s polarized light becomes f2(t) (f2 (t)≠f1 (t)). Here, respective f1 (t), f2 (t) are designated bynotations f1, f2 as follows.

HM 7 synthesizes output light from the measuring object 1 and otherlight branched by HM 3 and transmitted by an optical path OP3. Here,light transmitted by the optical path OP3 is reference light. PBS 8branches light multiplexed by HM 7 in 2 of light polarization planes ofwhich are perpendicular to each other.

A light receiving section 9 receives one light (for example, p polarizedlight) branched by PBS 8. A light receiving section 10 receives otherlight (for example, s polarized light) branched by PBS 8. A lightreceiving section 11 receives light multiplexed by PBS 5. Further, PBS 5receives light from a plane different from that emitted to the measuringobject 1.

Therefore, at the light receiving section 9, three kinds of light ofreference light (frequency f1′), emitted light polarized light(frequencies f1, f2) are interfered with each other. Further, referencelight is provided with a frequency f1′ different from that of signallight, which is produced by an optical length difference of an opticalpath branched by HM 3 to the optical path OP1, PBS 5, the measuringobject 1, HM 7 and an optical path of the optical path OP3, and theoptical length difference is sufficiently smaller than an optical pathlength difference of the optical path OP1 and the optical path OP2including the delay fiber 6. Therefore, a relationship of the frequencydifference is represented by |f1′−f2|>>|f1′−f1|.

Naturally, when the optical path difference between the optical pathbranched by HM 3 to the optical path OP1, PBS 5, the measuring object 1,HM 7 and the optical path of the optical path OP3=0, the frequencyf1=f1′.

Operation of the apparatus will be explained.

The wavelength variable light source 2 carries out wavelength sweep(frequency sweep) by a predetermined sweep speed. Further, HM 3 brancheslaser light from the wavelength variable light source in two. Further, apolarized wave controller, not illustrated, between the wavelengthvariable light source 2 and HM 3 pertinently controls polarized lightsuch that laser light is branched in two at PBS 4.

Further, PBS 4 branches laser light to be multiplexed by PBS 5 by way ofthe optical paths OP1, OP2. One of multiplexed light is outputted to themeasuring object 1 and other thereof is received by the light receivingsection 11.

HM 7 synthesizes output light (signal light) from the measuring object 1and other light (reference light) from the optical path OP3. Further,PBS 8 branches multiplexed interference light to two of linearlypolarized light polarization planes of which are perpendicular to eachother. Further, one light branched by PBS 8 is received by the lightreceiving section 9, other light is received by the light receivingsection 10.

Further, filtering is carried out by a filter, not illustrated, at arear stage and Jones matrix of the measuring object 1 is obtained bycalculating section, not illustrated. As objects to be filtered, forexample, at the light receiving section 9, there are present emitted ppolarized light of frequencies f1, f2 and an interference signal byreference light of the frequency f1′.

Therefore, in order to obtain respective elements of Jones matrix by anoutput signal from the light receiving section 9, it is necessary toextract an interference signal of emitted p polarized light of thefrequency f1 and reference light of the frequency f1′ and extract aninterference signal of emitted p polarized light of the frequency f2 andreference light of the frequency f1′. Therefore, by a low-pass filterfor passing a vicinity of a direct current component and a band-passfilter for passing a vicinity of a frequency difference |f1′−f2|,predetermined interference signals are provided and outputted tocalculating section, not illustrated, at the rear stage. Further, thecalculating section obtains Jones matrix. Further, by an output of thelight receiving section 11, nonlinearity of wavelength sweep of thewavelength variable light source 2 is corrected.

SECOND RELATED ART

FIG. 16 is a diagram showing a configuration of an opticalcharacteristic measuring apparatus of a second related art (refer to,for example, JP-A-2002-243585). In FIG. 16, a wavelength variable lightsource 2 outputs laser light while carrying out wavelength sweep by apredetermined wavelength sweep speed. A half mirror (hereinafter,abbreviated as HM) 3 branches laser light from the wavelength variablelight source 2 in two.

A polarized light delay section 6 c includes polarization beam splitters(hereinafter, abbreviated as PBS) 4 a, 4 b, and a delay fiber 6 forgenerating incident p polarized light, incident s polarized light fromone laser light branched by HM 3.

PBS 4 a branches laser light in two of light (p polarized light, spolarized light) polarization planes of which are perpendicular to eachother. Here, p polarized light is transmitted by an optical path OP1 ands polarized light is transmitted by an optical path OP2. PBS 4 bmultiplexes light branched by PBS 4 a and transmitted by the differentoptical paths OP1, OP2 to be outputted to a measuring object 1. Here,light inputted to the measuring object 1 is signal light. The delayfiber 6 is provided on the optical path OP2 between PBS 4 a, 4 b todelay s polarized light.

Therefore, since incident s polarized light passes through the delayfiber 6, when a frequency of incident p polarized light is designated bynotation f1 (t), a frequency of incident s polarized light becomes f2(t) (f2 (t)≠f1 (t)). Here, respective f1 (t), f2 (t) are designated bynotations f1, f2 as follows.

HM 7 multiplexes output light from the measuring object 1 and otherlight branched by HM 3 and transmitted by an optical path OP3 to beinterfered with each other. Here, light transmitted by the optical pathOP3 is reference light. PBS 8 branches light multiplexed by HM7 into twoof light polarization planes of which are perpendicular to each other.

A light receiving section 9 receives one light (for example, s polarizedlight) branched by PBS 8. A light receiving section 10 receives otherlight (for example, p polarized light) branched by PBS 8.

Therefore, explaining of the light receiving section 9, three kinds oflight of the reference light (frequency f1′), emitted s polarized light(frequencies f1, f2) are interfered with each other. Further, thereference light is provided with a frequency f1′ different from that ofthe signal light owing to an optical path length difference between thesignal light and the reference light.

That is, there is brought about an optical path length differencebetween an optical path branched by HM 3 to the optical path OP1, PBS 4b, the measuring object 1, HM 7 and the optical path OP3, the opticalpath length difference is sufficiently smaller than an optical pathlength difference between the optical path OP1 and the optical path OP2including the delay fiber 6. Therefore, a relationship of a frequencydifference is (|f1′−f2|>>|f1′−f1|).

Similarly, at the light receiving section 10, three kinds of light ofthe reference light (frequency f1′), emitted p polarized light(frequencies f1, f2) are interfered with each other.

Naturally, when the optical path difference between the optical pathbranched by HM 3 to the optical path OP1, PBS 4 b, the measuring object1, HM 7 and the optical path of the optical path OP3=0, the frequencyf1=f1′.

Filter circuits 101, 102 are provided at a rear stage of the lightreceiving sections 9, 10 for subjecting signals from the light receivingsection 9, 10 to low pass, band pass filtering. Calculating section 103is inputted with signals filtered by the filter circuits 101, 102,(signal after low pass filtering and signal after band pass filtering).

Operation of the apparatus will be explained.

The wavelength variable light source 2 carries out wavelength sweep(frequency sweep) by a predetermined sweep speed. Further, HM 3 brancheslaser light from the wavelength variable light source in two. Further, apolarized wave controller, not illustrated, between the wavelengthvariable light source 2 and HM 3 pertinently controls polarized lightsuch that laser light is branched in two at PBS 4 a.

Further, p polarized light, s polarized light transmitted by the opticalpaths OP1, OP2 to produce the frequency difference are multiplexed byPBS 4 b to be outputted to the measuring object 1.

HM 7 multiplexes output light (signal light) from the measuring object 1with other light (reference light) transmitted by the optical path OP3.Further, PBS 8 branches multiplexed interference light to two oflinearly polarized light polarization planes of which are perpendicularto each other. Further, one light branched by PBS 8 is received by thelight receiving section 9 and other light is received by the lightreceiving section 10.

Further, respective the filter circuits 101, 102 output signals from thelight receiving sections 9, 10, or signals subjected to lowpassfiltering, signals subjected to band pass filtering to the calculatingsection 103. Further, the calculating section 103 obtains Jones matrixof the measuring object 1 from an amplitude and a phase of aninterference signal after having been filtered.

Further, as signals to be filtered by the filter circuit 101, forexample, at the light receiving section 9, there is present aninterference signal by emitted s polarized light of frequencies f1, f2and reference light (s polarized light) of the frequency f1′.

Therefore, in order to obtain respective elements of Jones matrix by anoutput signal from the light receiving section 9, it is necessary toextract an interference signal of emitted s polarized light (frequencyf1) and the reference light (frequency f1′) and extract an interferencesignal of emitted s polarized light (frequency f2) and the referencelight (frequency f1′).

Hence, the filter circuit 101 carries out the separation by a low-passfilter for passing a vicinity of a direct current component and aband-pass filter for passing a vicinity of the frequency difference(|f1′−f2|) and outputs the separated interference signal to thecalculating section 103.

Similarly, as signals to be filtered by the filter circuit 102, forexample, at the light receiving section 10, there is present aninterference signal by emitted p polarized light of frequencies f1, f2and the reference light of the frequency f1′.

Therefore, in order to obtain respective elements of Jones matrix by theoutput signal from the light receiving section 10, it is necessary toobtain an interference signal of emitted p polarized light (frequencyf1) and the reference light (frequency f1′) and extract the interferencesignal of emitted p polarized light (frequency f2) and the referencelight (frequency f1′).

Hence, the filter circuit 102 carries out the separation by the low-passfilter for passing a vicinity of a direct current component and theband-pass filter for passing a vicinity of the frequency difference(|f1′−f2|) and outputs the separated interference signal to thecalculating section 103. Further, the calculating section 103 obtainsJones matrix from the interference signals from the filter circuits 101,102.

THIRD RELATED ART

FIG. 17 is a diagram showing a configuration of an opticalcharacteristic measuring apparatus of a third related art.

A wavelength variable light source 2 outputs laser light while carryingout wavelength sweep by a predetermined wavelength sweep speed. Anoptical fiber 366 transmits laser light from the wavelength variablelight source 2. A lens 466 makes laser light emitted from the opticalfiber 366 parallel light. A polarized wave controller 566 convertsparallel light from the lens 466 to a desired polarized state (forexample, linearly polarized light).

An interference section 666 includes a half mirror (hereinafter,abbreviated as HM) 666 a, mirrors 666 b, 666 c, a polarization beamsplitter (hereinafter, abbreviated as PBS) 666 d, a polarization planerotating section 666 e, polarizers 666 f, 666 g, branches light from thepolarized wave controller 566, inputs one branched light to themeasuring object 1, and makes output light (signal light) outputted fromthe measuring object 1 interfere with other branched light (referencelight).

HM 666 a is a branching section for branching parallel light from thepolarized wave controller 566 without depending on the polarized state,and outputs one branched light to the measuring object. The mirrors 666b, 666 c are arranged on an optical path of other branched lightbranched by HM 666 a, and successively reflect reference light.

PBS 666 d is arranged on an optical path of output light from themeasuring object 1, multiplexes reflected light (reference light) fromthe mirror 666 c and signal light to be branched in two of lightpolarization planes of which are orthogonal to each other.

The polarization plane rotating section 666 e is, for example, a ½ waveplate or the like, and provided between the mirror 666 b and the mirror666 c. The polarizer 666 f is provided on an optical path of onebranched light from PBS 666 d, and the polarizer 666 g is provided on anoptical path of other branched light from PBS 666 d for making signallight and reference light interfere with each other.

A photodiode 766 receives interference light from the polarizer 666 f ofthe interference section 666 and outputs a signal in accordance withoptical power (also referred to as optical intensity) of receivedinterference light. A photodiode 866 receives other interference lightfrom the polarizer 666 g of the interference section 666 and outputs asignal in accordance with the optical power of received interferencelight. A calculating section 966 is inputted with interference signalsfrom the photodiodes 766, 866.

Operation of the apparatus will be explained.

In order to input respective p polarized light and s polarized light tothe measuring object 1, wavelength sweep is carried out twice in apredetermined wavelength range. An explanation will be given from thefirst wavelength sweep.

The wavelength variable light source 2 outputs laser light whilecontinuously carrying out wavelength sweep in a predetermined wavelengthrange. Further, the lens 466 makes laser light transmitted by theoptical fiber 366 parallel light, and the polarized wave controller 566converts a polarized state of laser light made to be parallel light intop polarized light to be outputted to the interference section 666.

Further, HM 666 a branches light from the polarized wave controller 566,outputs one thereof to the measuring object 1 as signal light andoutputs other thereof to the mirror 666 b as reference light. Further,the polarization plane rotating section 666 e inclines a polarizationplane of reflected light from the mirror 666 b by 45° to be outputted tothe mirror 666 c such that optical power is uniformly branched at PBS666 d at a rear stage.

Further, PBS 666 d multiplexes output light (emitted s polarized light,emitted p polarized light in correspondence with incident p polarizedlight) from the measuring object 1 and reference light by way of themirrors 666 b, 666 c to be branched in two of light (p polarized light,s polarized light) polarization planes of which are orthogonal to eachother and outputs emitted p polarized light to the photodiode 766 andoutputs emitted s polarized light to the photodiode 866. Further,polarization planes of light (signal light and reference light)multiplexed and branched by PBS 666 d are orthogonal to each other andtherefore, the light is received by the photodiodes 766, 866 byinclining polarization planes by the polarizers 666 f, 666 g.

Thereby, the photodiode 766 is inputted with interference light ofsignal light operated by T₂₂ of Jones matrix and reference light.Further, the photodiode 866 is inputted with interference light ofsignal light operated by T₁₂ of Jones matrix and reference light.

Further, the photodiodes 766, 866 output electric signals in accordancewith optical power of received interference light to the calculatingsection 966.

Successively, second wavelength sweep is carried out and a point of thesecond wavelength sweep which differs from the first wavelength sweepresides in that the polarized wave controller 566 converts laser lightto s polarized light, that the photodiode 766 is inputted withinterference light of signal light operated by T₂₁ of Jones matrix andreference light, and that the photodiode 866 is inputted withinterference light of signal light operated by T11 of Jones matrix andreference light, the other operation is similar to that of the firstwavelength sweep and therefore, an explanation thereof will be omitted.

Further, the calculating section 966 calculates respective elements ofJones matrix from phases and amplitudes of interference signals based onrespective p polarized light, s polarized light to thereby calculate anoptical characteristic of the measuring object 1 from calculated Jonesmatrix.

WITH RESPECT TO THE FIRST RELATED ART

In the apparatus shown in FIG. 15, the frequency difference (|f1−f2|) ofincident p polarized light, incident s polarized light is determined bythe optical path length difference of the optical paths OP1, OP2 and thewavelength sweep speed (frequency sweep speed), and also a frequency ofa high frequency component of the interference signal is determined.

Therefore, the signal outputted from the light receiving section isseparated into an interference signal of a high frequency component(several tens through several hundreds [MHz]) and an interference signalof a direct current through low frequency component (which issufficiently lower than the high frequency component and is, forexample, about DC through 200 [kHz]).

However, it is very difficult currently to subject a total wavelengthrange to wavelength sweep with linearity. Therefore, owing tononlinearity of wavelength sweep, there poses a problem that awavelength difference (frequency difference |f1−f2|) of p polarizedlight, s polarized light passing through the optical paths OP1, OP2 doesnot stay to be constant, the frequency of the high frequency componentof the interference signal is varied and it is difficult to accuratelyobtain the optical characteristic.

Further, since the signal of the high frequency component is dealt with,in comparison with a case of dealing with the signal of the lowfrequency component, there poses a problem that circuit design of theband-pass filter for passing only the high frequency component, anelectric circuit at a rear stage of filter and the like is difficult,and a circuit configuration becomes complicated.

WITH RESPECT TO THE SECOND RELATED ART

According to the apparatus shown in FIG. 16, the frequency difference(|f1−f2|) of incident p polarized light, incident s polarized light isdetermined by the optical length difference of the optical path OP1, OP2and a wavelength sweep speed (frequency sweep speed) and also thefrequencies of the interference signals are determined.

Therefore, by filtering the signals outputted from the light receivingsections 9, 10 by the filter circuits 101, 102, an interference signalof a high frequency component (several tens through several hundreds[MHz]) and an interference signal of a direct current through a lowfrequency component (which is sufficiently lower than the high frequencycomponent and is, for example, about DC through 200 [kHz]) can beseparated.

However, it is currently very difficult for the wavelength variablelight source 2 to subject a total measuring wavelength range towavelength sweep with linearity. Therefore, owing to nonlinearity ofwavelength sweep, a wavelength difference (frequency difference |f1−f2|)of p polarized light, s polarized light respectively passing through theoptical paths OP1, OP2 does not stay to be constant, frequencies of theinterference signals are varied to pose a problem that it is difficultto accurately obtain an optical characteristic unless characteristics ofthe low-pass filter, the band-pass filter are changed (for example,passing frequency bands are made variable).

WITH RESPECT TO THE THIRD RELATED ART

Jones matrix of the measuring object 1 is calculated from phases andamplitudes of interference signals of signal light and reference light.

However, generally, with regard to a light wave interference signalmeasured by the photodiodes 766, 866, signal intensity proportional to atrigonometric function of a phase difference of multiplexed light(signal light, reference light) is obtained to pose a problem that it isdifficult to determine whether the phase difference is increased ordecreased.

Therefore, for example, in U.S. Pat. No. 6,376,830, there is constructeda configuration in which the phase difference of signal light andreference light is only increased or decreased by bringing the opticalpath of transmitting signal light, the optical path of transmittingreference light, the optical path of the measuring object 1 under apredetermined condition. Therefore, there poses a problem that aconfiguration of the optical characteristic measuring apparatus issignificantly restricted, that is, an optical path length of themeasuring object 1 is limited.

SUMMARY OF THE INVENTION

The present invention has been made in view of the above circumstances,and provides an optical characteristic measuring apparatus capable ofaccurate measurement even when a frequency difference of first andsecond input lights is varied.

Further, the present invention provides an optical characteristicmeasuring apparatus capable of accurate measurement even when afrequency sweep speed of a wavelength variable light source is notconstant.

Further, the present invention provides an optical characteristicmeasuring apparatus capable of easily determining an increase or adecrease of a phase difference of light (signal light and referencelight) to be multiplexed.

In some implementations, an optical characteristic measuring apparatusof the invention for measuring an optical characteristic of a measuringobject comprises:

a light source section which sweeps wavelengths of a first input lightand a second input light respectively, frequencies of the first inputlight and the second input light being different from each other andpolarized states of the first input light and the second input lightbeing perpendicular to each other, and outputs the first input light andthe second input light;

an interference section which branches each of the first input light andthe second input light from the light source section, inputs onebranched light to the measuring object, makes output light from themeasuring object interfere with other branched light, and outputs aplurality of interference lights;

a plurality of light receiving sections which are respectively providedfor the interference lights outputted from the interference section,receives the interference lights respectively, and outputs signals inaccordance with optical powers of the interference lights respectively;and

a low-pass filter for filtering the signals outputted from the lightreceiving sections,

wherein the plurality of interference lights includes:

a first interference light in which the first input light interfereswith an output light in a first polarized state of the output light fromthe measuring object;

a second interference light in which the second input light interfereswith the output light in the first polarized state of the output lightfrom the measuring object;

a third interference light in which the first input light interfere withan output light in a second polarized state of the output light from themeasuring object; and

a fourth interference light in which the second input light interfereswith the output light in the second polarized state of the output lightfrom the measuring object, and

the first polarized state of the output light and the second polarizedstate of the output light are perpendicular to each other.

According to the optical characteristic measuring apparatus, theinterference section outputs the interference light of the first inputlight and the light in the first polarized state in the output light ofthe measuring object (the output light in correspondence with the firstand the second input light), the interference light of the second inputlight and the light in the first polarized state in the output light ofthe measuring object, the interference light of the first input lightand the light in the second polarized state (perpendicular to the firstpolarized state) in the output light of the measuring object, and theinterference light of the second input light and the light in the secondpolarized state in the output light (the output light in correspondencewith the first and the second input light) of the measuring object.Further, a signal based on the interference light is filtered by thelow-pass filter. Thereby, an interference signal of the a low frequencycomponent passing through the low-pass filter is not influenced by thedifference of the frequencies of the first and the second input lightproduced by nonlinearity of frequency sweep of the light source section.Therefore, even when the difference of the frequencies of the first andthe second input light is varied, the interference signal can accuratelybe measured. Further, only the signal of the low frequency component isdealt with and therefore, circuit design of the low-pass filter, anelectric circuit at a rear stage of the filter and the like isfacilitated and a circuit configuration is simplified.

In the optical characteristic measuring apparatus of the invention, thelow-pass filter passes a signal having a frequency lower than adifference of the frequencies of the first input light and the secondinput light which are outputted from the light source section.

In the optical characteristic measuring apparatus of the invention, thefirst input light and the second input light are linearly polarizedlights.

In the optical characteristic measuring apparatus of the invention, theinterference section includes:

a multiplexing section for multiplexing the first input light and thesecond input light from the light source section;

an input light branching section for branching a multiplexed light andoutputting one branched light to the measuring object;

a first polarization beam splitter for branching the output light fromthe measuring object;

a second polarization beam splitter for multiplexing and branching onebranched light of the first polarization beam splitter and otherbranched light of the input light branching section;

a third polarization beam splitter for multiplexing and branching otherbranched light of the first polarization beam splitter and otherbranched light of the input light branching section; and

a plurality of polarizers which are provided for branched lights of thesecond polarization beam splitter and branched lights of the thirdpolarization beam splitter respectively, and respectively makesmultiplexed lights of the branched lights of the second polarizationbeam splitter and multiplexed lights of the branched lights of the thirdpolarization beam splitter interfere.

In the optical characteristic measuring apparatus of the invention, theinterference section includes:

a first input light branching section for branching the first inputlight;

a second input light branching section for branching the second inputlight;

a multiplexing section for multiplexing one branched light from thefirst input light branching section and one branched light from thesecond input light branching section so as to output a multiplexed lightto the measuring object;

an output light branching section for branching the output light fromthe measuring object;

a first polarization beam splitter for multiplexing and branching onebranched light of the output light branching section and other branchedlight of the first input light branching section;

a second polarization beam splitter for multiplexing and branching otherbranched light of the output light branching section and other branchedlight of the second input light branching section; and

a plurality of polarizers which are provided for branched lights of thefirst polarization beam splitter and branched lights of the secondpolarization beam splitter respectively, and respectively makesmultiplexed lights of the branched lights of the first polarization beamsplitter and multiplexed lights of the branched lights of the secondpolarization beam splitter interfere.

In the optical characteristic measuring apparatus of the invention, theinterference section includes:

a first input light branching section for branching the first inputlight;

a second input light branching section for branching the second inputlight;

a multiplexing section for multiplexing one branched light from thefirst input light branching section and one branched light from thesecond input light branching section so as to output a multiplexed lightto the measuring object;

an output light branching section for branching the output light fromthe measuring object;

a first output light multiplexing section for multiplexing one branchedlight of the output light branching section and other branched light ofthe first input light branching section so as to make the one branchedlight of the output light branching section interfere with the otherbranched light of the first input light branching section;

a first polarization beam splitter for branching interference light ofthe first output light multiplexing section;

a second output light branching section for multiplexing other branchedlight of the output light branching section and other branched light ofthe second input light branching section so as to make the otherbranched light of the output light branching section interfere with theother branched light of the second input light branching section; and

a second polarization beam splitter for branching interference light ofthe second output light multiplexing section.

In the optical characteristic measuring apparatus of the invention, thefirst output light multiplexing section and the second output lightmultiplexing section are commonly provided in one section.

Accordingly, the first and the second output light multiplexing sectionsare made common and therefore, a number of parts is reduced to achievesmall-sized formation, facilitating alignment, a reduction in cost.

In the optical characteristic measuring apparatus of the invention, thefirst input light branching section and the second input light branchingsection are commonly provided in one section.

Accordingly, the first and the second input light branching sections aremade common and therefore, a number of parts is reduced to achievesmall-sized formation, facilitating alignment, a reduction in cost.

In the optical characteristic measuring apparatus of the invention, thefirst polarization beam splitter and the second polarization beamsplitter are commonly provided in one polarization beam splitter.

Accordingly, the first and the second polarization beam splitters aremade common and therefore, a number of parts is reduced to achievesmall-sized formation, facilitating alignment, a reduction in cost.

The optical characteristic measuring apparatus of the invention furthercomprises:

a plurality of polarizers which is provided for branched lights of thefirst polarization beam splitter and branched lights of the secondpolarization beam splitter respectively, and respectively passes onlyinterference lights having a predetermined polarization plane.

Accordingly, the polarizer passes only light of a predeterminedpolarization plane to be outputted to the light receiving section andtherefore, noise of the interference signal can be reduced.

In the optical characteristic measuring apparatus of the invention, theinterference section is an interferometer of a spatial light type.

Accordingly, by constituting the interference section by theinterferometer of the spatial light type, an optical system can bedownsized and is made to be strong at vibration.

In some implementations, an optical characteristic measuring apparatusof the invention for measuring an optical characteristic of a measuringobject comprises:

a first wavelength variable light source which sweeps a wavelength of afirst input light and outputs the first input light to an interferencesection;

a second wavelength variable light source which sweeps a wavelength of asecond input light and outputs the second input light to theinterference section, frequencies of the first input light and thesecond input light being different from each other and polarized statesof the first input light and the second input light being perpendicularto each other;

the interference section which multiplexes and inputs the first inputlight and the second input light to the measuring object, makes outputlight from the measuring object interfere with at least one of the firstinput light and the second input light, and outputs a plurality ofinterference lights;

a detecting section for detecting a frequency difference of the firstinput light and the second input light from the first wavelengthvariable light source and the second wavelength variable light source;and

a control section for controlling a frequency difference of the firstwavelength variable light source and the second wavelength variablelight source based on the frequency difference detected by the detectingsection.

Accordingly, the detecting section detects the frequency difference oflight outputted from the first and the second wavelength variable lightsources, the control section controls the wavelength sweep speed of theat least one of the first and the second wavelength variable lightsources based on the frequency difference detected by the detectingsection and therefore, one light source is subjected to wavelength sweepwhile maintaining the constant light frequency difference relative toother light source. Thereby, a center frequency of the interferencelight outputted from the interference section is not varied. Therefore,the optical characteristic of the measuring object can accurately bemeasured even when the wavelength sweep speed of the wavelength variablelight source is not constant.

In some implementations an optical characteristic measuring apparatus ofthe invention for measuring an optical characteristic of a measuringobject comprises:

a wavelength variable light source which sweeps a wavelength of a laserlight and outputs the laser light;

a branching section which branches the laser light from the wavelengthvariable light source and outputs one branched light to an interferencesection as a first input light;

the interference section which multiplexes and inputs the first inputlight and a second input light to the measuring object, makes outputlight from the measuring object interfere with at least one of the firstinput light and the second input light, and outputs a plurality ofinterference lights, frequencies of the first input light and the secondinput light being different from each other and polarized states of thefirst input light and the second input light being perpendicular to eachother; and

an acousto-optical modulator which shifts a frequency of other branchedlight from the branching section for a predetermined amount and outputsthe frequency-shifted other branched light to the interference sectionas the second input light.

Accordingly, the acousto-optical modulator shifts the laser lightoutputted from the wavelength variable light source by a predeterminedamount to be outputted to the interference section and therefore, theinterference section is inputted with the first and the second inputlight maintaining the constant light frequency difference. Thereby, thecenter frequency of the interference light outputted from theinterference section is not varied. Therefore, the opticalcharacteristic of the measuring object can accurately be measured evenwhen the wavelength sweep speed of the wavelength variable light sourceis not constant.

In the optical characteristic measuring apparatus of the invention, atleast one of the first wavelength variable light source and the secondwavelength variable light source includes a vertical-cavitysurface-emitting laser (VCSEL) forming a resonator by a movable mirrorbeing formed by a semiconductor micromachining technology.

In the optical characteristic measuring apparatus of the invention, thewavelength variable light source includes a vertical-cavitysurface-emitting laser (VCSEL) forming a resonator by a movable mirrorbeing formed by a semiconductor micromachining technology.

Accordingly, the wavelength variable light source outputs the laserlight by using the vertical-cavity surface-emitting laser (VCSEL)forming the resonator by the movable mirror formed by the semiconductormicromachining technology and therefore, cost can be reduced and thewavelength sweep speed can be accelerated. Thereby, a number of times ofwavelength sweep is increased in a predetermined time period and anaveraging processing can be increased and an accuracy of measurement ispromoted. Further, although the interferometer of the interferencesection is much liable to be effected with an influence of disturbance(vibration), by shortening the wavelength sweep time period, theinfluence of disturbance can be restrained and the accuracy ofmeasurement is promoted.

In the optical characteristic measuring apparatus of the invention, atleast one of the first wavelength variable light source and the secondwavelength variable light source includes a surface emitting laserforming a resonator by a movable mirror being formed by a semiconductormicromachining technology, and

the first wavelength variable light source and the second wavelengthvariable light source are provided on a same substrate.

Accordingly, the wavelength variable light source outputs the laserlight by using the surface emitting laser forming the resonator by themovable mirror formed by the semiconductor micromachining technology andtherefore, cost can be reduced and the wavelength sweep speed can beaccelerated. Thereby, a number of times of wavelength sweep is increasedin a predetermined time period and an averaging processing can beincreased and an accuracy of measurement is promoted. Further, althoughthe interferometer of the interference section is much liable to beeffected with an influence of disturbance (vibration), by shortening thewavelength sweep time period, the influence of disturbance can berestrained and the accuracy of measurement is promoted.

In the optical characteristic measuring apparatus of the invention, theinterference section includes a polarization beam splitter whichmultiplexes the first input light and the second input light and outputsa multiplexed light to the measuring object.

Accordingly, the polarization beam splitter of the interference sectionmultiplexes the first and the second input light to be outputted to themeasuring object and therefore, the first and the second input light canefficiently be multiplexed. Thereby, loss of optical power can berestrained and interference light having strong optical power can beprovided.

In the optical characteristic measuring apparatus of the invention, theinterference section includes:

a polarization beam splitter which multiplexes at least one of the firstinput light and the second input light with the output light from themeasuring object, and branches a multiplexed light into s polarizedlight and p polarized light; and

a polarization plane rotating section which inclines at least onepolarization plane of the first input light and the second input lightby 45°, and outputs an inclined light to the polarization beam splitter.

The optical characteristic measuring apparatus of the invention furthercomprising:

a plurality of light receiving sections which are respectively providedfor the interference lights outputted from the interference section,receives the interference lights respectively, and outputs signals inaccordance with optical powers of the interference lights respectively;and

a low-pass filter for filtering the signals outputted from the lightreceiving sections,

wherein the interference section branches each of the first input lightand the second input light, inputs one branched light to the measuringobject, makes the output light from the measuring object interfere withother branched light, and outputs the plurality of interference lights,

the plurality of interference lights includes:

a first interference light in which the first input light interfereswith an output light in a first polarized state of the output light fromthe measuring object;

a second interference light in which the second input light interfereswith the output light in the first polarized state of the output lightfrom the measuring object;

a third interference light in which the first input light interfere withan output light in a second polarized state of the output light from themeasuring object; and

a fourth interference light in which the second input light interfereswith the output light in the second polarized state of the output lightfrom the measuring object, and

the first polarized state of the output light and the second polarizedstate of the output light are perpendicular to each other.

Accordingly, the interference section constitutes the interferencesignal for obtaining respective elements of Jones matrix by theinterference signal of the low frequency to be outputted. Thereby, aninfluence of the frequency difference of the first and the second inputlight produced by nonlinearity of frequency sweep of the wavelengthvariable light source can further be alleviated. Therefore, even whenthe frequency difference of the first and the second input light isvaried, the interference signal can accurately be measured. Further,only the signal of the low frequency component is dealt with andtherefore, circuit design of the low-pass filter, an electric circuit orthe like at a rear stage of the filter is facilitated and a circuitconfiguration is simplified.

In the optical characteristic measuring apparatus of the invention, thelow-pass filter passes a signal having a frequency lower than adifference of the frequencies of the first input light and the secondinput light.

In the optical characteristic measuring apparatus of the invention, theinterference section is an interferometer of a spatial light type.

Accordingly, by constituting the interference section by theinterferometer of the spatial light type, the optical system can bedownsized and is made to be strong at vibration.

In some implementations, an optical characteristic measuring apparatusof the invention for measuring an optical characteristic of a measuringobject comprises:

an interference section which branches light from a light sourcesection, inputs one branched light to the measuring object, and makesother branched light interfere with output light being outputted fromthe measuring object so as to form interference fringes by multiplexingthe output light and the other branched light while inclining an opticalaxis of the output light and an optical axis of the other branchedlight,

wherein a moving direction and a moving amount of the interferencefringes are measured.

Accordingly, the interference fringes are formed by multiplexing theoutput light (signal light) of the measuring object and the otherbranched light (reference light) by shifting the optical axis of theother branched light relative to the optical axis of the output light(signal light) and the moving amount and the moving direction theinterference fringes are measured. That is, the interference fringes aremoved in a predetermined direction in accordance with an increase or adecrease in a phase difference of light (signal light and referencelight) to be multiplexed. Thereby, the increase or the decrease in thephase difference of light to be multiplexed can easily be determined.

The optical characteristic measuring apparatus of the invention furthercomprises:

at least one photodiode array which includes a plurality of photodiodesand receives an interference light from the interference section, thephotodiodes being arranged to be shifted along a direction in which theinterference fringes are formed; and

an interference signal converting section which generates aninterference signal from an output of the photodiode array, a phase ofthe interference signal being shifted.

Accordingly, the interference section forms the spatial interferencefringes by making the output light (signal light) and the branched light(reference light) interfere with each other by inclining the opticalaxis of the output light and the optical axis of the other branchedlight. Further, the light is received by the plurality of photodiodesthe phase of which are shifted relative to the period of theinterference fringes, and an interference signal converting sectionoutputs the interference signals the phases of which are shifted fromeach other by 90° based on the output from the photodiode. Thereby, themoving direction and the moving amount the interference fringes arecalculated and the increase or the decrease in the phase difference oflight (signal light and reference light) to be multiplexed can easily bedetermined. Therefore, the optical path length of the measuring objectis not restricted.

In the optical characteristic measuring apparatus of the invention, theplurality of photodiodes includes at least four photodiodes, and

the respective photodiodes receive light by equally dividing one spatialperiod of the interference fringes by four.

In the optical characteristic measuring apparatus of the invention, theinterference signal converting section outputs a subtraction result ofan output of a third photodiode of the photodiodes and an output of afirst photodiode of the photodiodes as a first interference signal, and

the interference signal converting section outputs a subtraction resultof an output of a fourth photodiode of the photodiodes and an output ofa second photodiode of the photodiode as a second interference signal.

In the optical characteristic measuring apparatus of the invention, theat least one photodiode array includes a plurality of photodiode arrayswhich are arranged along a direction in which the interference fringesare formed.

Accordingly, the plurality of pieces of photodiode arrays are providedalong the direction of aligning the interference fringes, and theinterference signal converting section generates the interferencesignals from the outputs of the plurality of photodiode arrays. Thereby,even when a nonuniformity (random noise) is present at a section or atotal of the interference fringes, the interference signal which is lessinfluenced by the nonuniformity can be provided by averaging.

In the optical characteristic measuring apparatus of the invention, theplurality of photodiodes includes at least (4×n) photodiodes,

the respective photodiodes receive light by equally dividing one spatialperiod of the interference fringes by four,

the interference signal converting section outputs a subtraction resultof an output of (4×(i−1)+3)-th photodiode of the photodiodes and anoutput of (4×(i−1)+1)-th photodiode of the photodiodes as a firstinterference signal, and

the interference signal converting section outputs a subtraction resultof an output of (4×(i−1)+4)-th photodiode of the photodiodes and anoutput of (4×(i−1)+2)-th photodiode of the photodiodes as a secondinterference signal,

where notations n, i designate natural numbers.

Accordingly, the photodiode array measures the plurality of periods ofinterference fringes, and the interference signal converting sectiongenerates the interference signals from the outputs of the photodiodearrays. Thereby, even when there is a nonuniformity (random noise) at asection or a total of the interference fringes, the interference signalswhich are less influenced by the nonuniformity can be provided byaveraging.

The optical characteristic measuring apparatus of the invention furthercomprises:

a correcting section for correcting a difference between a spatialperiod of the interference fringes and a period of the photodiodes ofthe photodiode array.

Accordingly, the correcting section corrects the error in the movingamount produced by the shift of the photodiode array of the spatialperiod of the interference fringes and therefore, the moving amount canaccurately be measured.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an entire configuration diagram of an apparatus showing afirst embodiment of the invention.

FIG. 2 is a configuration diagram showing a first example of aninterference section provided in the apparatus shown in FIG. 1.

FIG. 3 is a configuration diagram showing a second example of theinterference section provided in the apparatus shown in FIG. 1.

FIG. 4 is a configuration diagram showing a third example of theinterference section provided in the apparatus shown in FIG. 1.

FIG. 5 is a configuration diagram showing a fourth example of theinterference section provided in the apparatus shown in FIG. 1.

FIG. 6 is a configuration diagram showing a second embodiment of theinvention.

FIG. 7 is a configuration diagram showing a third embodiment of theinvention.

FIG. 8 is a configuration diagram showing a fourth embodiment of theinvention.

FIG. 9 is a configuration diagram showing a fifth embodiment of theinvention.

FIG. 10 is a configuration diagram showing a sixth embodiment of theinvention.

FIG. 11 is a diagram showing an essential section of the apparatus shownin FIG. 10.

FIG. 12 is a configuration diagram showing a seventh embodiment of theinvention.

FIG. 13 is a configuration diagram showing a eighth embodiment of theinvention.

FIG. 14 is a diagram showing an input/output characteristic of ameasuring object.

FIG. 15 is a diagram showing a configuration of an opticalcharacteristic measuring apparatus of a related art.

FIG. 16 is a diagram showing a configuration of an opticalcharacteristic of a measuring apparatus of a related art.

FIG. 17 is a diagram showing a configuration of optical characteristicmeasuring apparatus of a related art.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

An embodiment of the invention will be explained in reference to thedrawings as follows.

FIG. 1 is a configuration diagram showing a first embodiment of theinvention. Here, sections the same as those of FIG. 14, FIG. 15 areattached with the same notations and an explanation thereof will beomitted. In FIG. 1, a light source section 13 sweeps wavelengths(frequency sweep) of respective incident p polarized light (first inputlight) and incident s polarized light (second input light) of whichfrequencies (light frequencies) differ from each other and of whichpolarized states are perpendicular to each other so as to output theincident p polarized light and the incident s polarized light. Here, afrequency of incident p polarized light is designated by notation f1,and a frequency of incident s polarized light is designated by notationf2 (f1≠f2).

An interference section 14 branches respective incident s polarizedlight, incident p polarized light from the light source section 13 andinputs one branched light to the measuring object 1 as signal light.Output light (signal light) from the measuring object 1 with regard toincident p polarized light, incident s polarized light is interferedwith other branched light (reference light) to output a plurality ofinterference light. Further, interference lights outputted by theinterference section 14 are (a) through (d) shown below.

(a) Incident p polarized light of reference light and emitted ppolarized light of signal light.

(b) Incident s polarized light of reference light and emitted ppolarized light of signal light.

(c) Incident p polarized light of reference light and emitted spolarized light of signal light.

(d) Incident s polarized light of reference light and emitted spolarized light of signal light.

Further, signal light and reference light at the interference section 14are multiplexed naturally after having been transmitted by differentoptical paths. Here, when frequencies of reference light immediatelybefore being multiplexed with signal light are designated by notationsf1′, f2′, a frequency difference of first, second input light (|f1−f2|)outputted by the light source section 13 is set to be sufficientlylarger than frequency differences (|f2−f2′|, |f1−f1′|) produced by anoptical length difference of signal light and reference light.

For example, the frequency differences (|f2−f2′|, |f1−f1′|) are about 0through 200 [kHz], and the frequency difference (|f1−f2|) by the lightsource section 13 is about several tens through several hundreds [MHz].Naturally, optical lengths of signal light and reference light may bethe same.

Light receiving sections 15 through 18 are provided for respectiveinterference light outputted from the interference section 14 forreceiving interference light and outputting signals in accordance withoptical power of interference light.

Low-pass filters 19 through 22 are provided for the respective lightreceiving sections 15 through 18 for filtering signals outputted fromthe respective light receiving sections 15 through 18 and passing onlysignals of frequency components lower than the frequency difference(|f1−f2|) of incident s polarized light and incident p polarized light.

Operation of the apparatus will be explained.

The light source section 13 subjects incident s polarized light,incident p polarized light to wavelength sweep in a predeterminedwavelength range to be outputted to the interference section 14.

The interference section 14 branches incident s polarized light,incident p polarized light and outputs one thereof to the measuringobject 1 as signal light. Further, output light from the measuringobject 1 returns to the interference section 14. That is, emitted spolarized light and emitted p polarized light with regard to incident spolarized light return, emitted s polarized light and emitted ppolarized light with regard to incident p polarized light return.Further, the interference section 14 synthesizes emitted s polarizedlight, emitted p polarized light from the measuring object 1 andincident s polarized light, incident p polarized light of referencelight to be interfered with each other.

Further, the interference section 14 outputs interference light ofemitted s polarized light (frequencies f1, f2) and incident s polarizedlight (f2′) to the light receiving section 15, outputs interferencelight of emitted s polarized light (frequencies f1, f2) and incident ppolarized light (f1′) to the light receiving section 16, outputsinterference light of emitted p polarized light (frequencies f1, f2) andincident s polarized light (f2′) to the light receiving section 17,outputs interference light of emitted p polarized light (frequencies f1,f2) and incident p polarized light (f1′) to the light receiving section18.

Further, the respective light receiving sections 15 through 18 outputsignals in accordance with optical power of interference light to thelow-pass filters 19 through 22. Further, the low-pass filters 19 through22 pass signals of low frequency components (for example, about DCthrough 200 [kHz]) of the interference signals outputted from the lightreceiving sections 15 through 18 to be outputted to calculating section,not illustrated, at a rear stage.

Specifically, the light receiving section 15 is inputted with emitted spolarized light (frequencies f1, f2), that is, signal light operated byT₁₁, T₁₂ of Jones matrix and reference light. Therefore, by filteringthe interference signal from the light receiving section 15 by thelow-pass filter 19, as the interference signal (incident s polarizedlight of frequency f2′, emitted s polarized light of frequency f2) afterhaving been filtered, only the interference signal operated only by T₁₁of Jones matrix is extracted.

Similarly, as the interference signals after having been filtered byrespective the low-pass filters 20 through 22, only the interferencesignals operated only by T₁₂, T₂₁, T₂₂ of Jones matrix are extracted.

Further, calculating section, not illustrated, at the rear stage obtainsrespective elements of Jones matrix from amplitudes and phases of theinterference signals constituting output signals from the low-passfilters 19 through 22, and obtains an optical characteristic of themeasuring object 1 from Jones matrix.

In this way, the interference section 14 outputs interference light ofincident p polarized light and emitted s polarized light, interferencelight of polarized light of incident p polarized light and emitted ppolarized light, interference light of incident s polarized light andemitted s polarized light, interference light of polarized light ofincident s polarized light and emitted p polarized light, theinterference signals are filtered by the low-pass filters 19 through 22.Thereby, the interference signals of low frequency components passingthrough the low-pass filter 19 through 22 are not effected with aninfluence of the frequency difference of incident s polarized light andincident p polarized light produced by nonlinearity of frequency sweepof the light source section 13. Therefore, even when the frequencydifference of incident p polarized light and incident s polarized lightis varied, the interference signal can accurately be measured. Further,only signals of the low frequency components are dealt with andtherefore, circuit design of the low-pass filters 19 through 22,electric circuits at rear stages of the filters and the like isfacilitated and the circuit configuration is simplified.

First Example of Interference Section

Next, FIG. 2 is a configuration diagram showing a first example of theinterference section 14 in the apparatus shown in FIG. 1. Here, sectionsthe same as those of FIG. 1 are attached with the same notations and anexplanation thereof will be omitted. In FIG. 2, the interference section14 includes a multiplexing section 14 a, a branching section 14 b, PBS14 c through 14 e, wave plates 14 f, 14 g and polarizers 14 h through 14k.

Further, the branching section 14 b branches light without depending ona polarized state and is, for example, HM, a non-polarization beamsplitter, an optical fiber coupler or the like. Further, the wave plates14 f, 14 g are, for example, ½ wave plates or the like.

The multiplexing section 14 a is, for example, a non-polarization beamsplitter, HM, an optical fiber coupler or the like for multiplexingincident s polarized light, incident p polarized light from the lightsource section 13.

The input light branching section 14 b branches light multiplexed by themultiplexing section 14 a and inputs first branched light to themeasuring object as signal light. Further, second, third branched lightconstitute reference light.

First PBS 14 c branches output light from the measuring object 1 toemitted s polarized light, emitted p polarized light. The wave plate 14f is provided between PBS 14 c and PBS 14 d for inclining a polarizationplane of emitted s polarized light by 45°. The wavelength plate 14 g isprovided between PBS 14 c and PBS 14 e for inclining a polarizationplane of emitted p polarized light by 45°.

Second PBS 14 d synthesizes second branched light (reference light) fromthe branching section 14 c and emitted s polarized light from the waveplate 14 f to be branched in two of lights of polarization planes ofwhich are perpendicular to each other to be outputted to the lightreceiving sections 15, 16.

Third PBS 14 e synthesizes third branched light (reference light) fromthe branching section 14 c and emitted p polarized light from the waveplate 14 g to be branched into two of light polarization planes of whichare perpendicular to each other to be outputted to the light receivingsections 17, 18.

Respective the polarizers 14 h through 14 k are provided for respectivebranched light of PBS 14 d, 14 e, that is, provided between PBS 14 d andthe light receiving section 15, between PBS 14 d and the light receivingsection 16, between PBS 14 e and the light receiving section 17, betweenPBS 14 e and the light receiving section 18.

Operation of the apparatus will be explained.

The multiplexing section 14 a synthesizes incident s polarized light(frequency f2), incident p polarized light (frequency f1) from the lightsource section 13. Further, the branching section 14 b branchespolarized light to signal light and reference light and outputs signallight to the measuring object 1. Here, emitted s polarized light(frequencies f1, f2), emitted p polarized light (frequencies f1, f2) areoutputted from the measuring object 1, emitted s polarized light isoperated by T₁₁, T₁₂, emitted p polarized light is operated by T₂₁, T₂₂.

Further, PBS 14 c branches polarized light to emitted s polarized light,emitted p polarized light. A polarization plane of the branched emitteds polarized light is inclined by 45° by the wave plate 14 f, thereafter,the branched emitted s polarized light is multiplexed with referencelight (incident p polarized light (frequency f1′), incident s polarizedlight (frequency f2′)) at PBS 14 d and branched to light polarizationplanes of which are perpendicular to each other.

Thereby, one branched light of branched light outputted from PBS 14 d islight constituted by multiplexing incident s polarized light (frequencyf2′) and emitted s polarized light (frequencies f1, f2) and otherbranched light thereof constitutes light multiplexed with incident ppolarized light (frequency f1′) and emitted s polarized light(frequencies f1,f2).

Further, polarization planes of light multiplexed and branched at PBS 14d are perpendicular to each other and therefore, light multiplexed andbranched by PBS 14 d are interfered with each other by incliningpolarization planes by the polarizers 14 h, 14 i to be received by thelight receiving sections 15, 16.

Similarly, a polarization plane of emitted p polarized light branched byPBS 14 c is inclined by 45° by the wave plate 14 g, thereafter, emittedp polarized light branched by PBS 14 c is multiplexed with referencelight (incident p polarized light (frequency f1′), incident s polarizedlight (frequency f2′)) at PBS 14 e, thereafter, branched to lightpolarization planes of which are perpendicular to each other.

Thereby, one branched light of branched light outputted from PBS 14 e islight constituted by multiplexing incident s polarized light (frequencyf2′) and emitted p polarized light (frequencies f1, f2) and otherbranched light becomes light multiplexed with incident p polarized light(frequency f1′) and emitted P polarized light (frequencies f1, f2).

Further, polarization planes of light multiplexed and branched by PBS 14e are perpendicular to each other and therefore, light multiplexed andbranched by PBS 14 e are interfered with each other by inclining thepolarization planes by the polarizers 14 j, 14 k to be received by thelight receiving sections 17, 18.

Further, output signals from the light receiving sections 15 through 18are filtered respectively by the low-pass filters 19 through 22 andrespective signals after having been filtered constitute interferencesignals operated only by T₁₁, T₁₂, T₂₁, T₂₂ of Jones matrix. Further,operation other than the above-described is similar to that of theapparatus shown in FIG. 1 and therefore, an explanation thereof will beomitted.

Second Example of Interference Section

Next, FIG. 3 is a configuration diagram showing a second example of theinterference section of the apparatus shown in FIG. 1. Here, sectionsthe same as those of FIG. 1 are attached with the same notations and anexplanation thereof will be omitted. In FIG. 3, an interference section23 is provided in place of the interference section 14, the interferencesection 23 outputs interference light similar to that of theinterference section 14 and includes branching sections 23 a trough 23c, a multiplexing section 23 d, PBS 23 e, 23 f, wave plates 23 g, 23 h,polarizers 23 i through 23 l.

Further, the branching sections 23 a through 23 c branch light withoutdepending on a polarized state and are, for example, HM,non-polarization beam splitters, optical fiber couplers or the like.Further, the wave plates 23 g, 23 h are, for example, ½ plates or thelike.

The first input light branching section 23 a branches incident ppolarized light (frequency f1) from the light source section 13 in two,outputs one thereof to the multiplexing section 23 d as signal light andoutputs other thereof to the wave plate 23 g as reference light.

The second input light branching section 23 b branches incidentspolarized light (frequency f2) from the light source section 13 in two,outputs one thereof to the multiplexing section 13 d as signal light andoutputs other thereof to the wave plate 23 h as reference light.

The multiplexing section 23 d is, for example, PBS, a non-polarizationbeam splitter, HM, an optical fiber coupler or the like for multiplexingsignal light from the branching section 23 a, 23 b to be outputted tothe measuring object 1.

The output light branching section 23 c branches output light (signallight) from the measuring object 1 in two, outputs one thereof to PBS 23e and outputs other thereof to PBS 23 f.

The wave plate 23 g is provided between the branching section 23 a andPBS 23 e and inclines a polarization plane of incident p polarized lightof reference light by 45°.

First PBS 23 e synthesizes one branched light (signal light) from thebranching section 23 c and incident p polarized light of reference lightfrom the wave plate 23 g to be branched in two of light polarizationplanes of which are perpendicular to each other to be outputted to thelight receiving sections 16, 18.

Second PBS 23 f synthesizes other branched light (signal light) from thebranching section 23 c and incident s polarized light of reference lightfrom the wave plate 23 h to be branched in two of light polarizationplanes of which are perpendicular to each other to be outputted to thelight receiving sections 15, 17.

Respective the polarizers 23 i through 23 l are provided for respectivebranched light of PBS 23 e, 23 f, that is, provided between PBS 23 f andthe light receiving section 15, between PBS 23 e and the light receivingsection 16, between PBS 23 f and the light receiving section 17, betweenPBS 23 e and the light receiving section 18.

Operation of the apparatus will be explained.

The branching section 23 a branches incident p polarized light(frequency f1) from the light source section 13 in two, outputs onethereof to the multiplexing section 23 d as signal light and outputsother thereof to the wave plate 23 g as reference light. Further, thebranching section 23 b branches incidents polarized light (frequency f2)from the light source section 13 in two, outputs one thereof to themultiplexing section 23 d as signal light and outputs other thereof tothe wave plate 23 h as reference light. Further, respective the waveplates 23 g, 23 h incline polarization planes of reference light by 45°.

Further, the multiplexing section 23 d synthesizes signal light from thebranching sections 23 a, 23 b to be outputted to the measuring object 1.Further, when the multiplexed section 23 d synthesizes light by usingPBS, the multiplexing section 23 d can synthesize light more efficientlythan an optical element (for example, HM, non-polarization beamsplitter, optical fiber coupler or the like) for multiplexing andbranching light without depending on a polarized state.

Further, the branching section 23 c branches emitted s polarized light(frequencies f1, f2), emitted p polarized light (frequencies f1, f2)outputted from the measuring object 1, outputs one thereof to PBS 23 eand outputs other thereof to PBS 23 f. Naturally, emitted s polarizedlight is operated by T₁₁, T₁₂, emitted p polarized light is operated byT₂₁, T₂₂.

Further, PBS 23 e synthesizes reference light (incident p polarizedlight (frequency f1′)) from the wave plate 23 g and signal light(emitted s polarized light (frequencies f1, f2), emitted p polarizedlight (frequencies f1, f2)) to be thereafter branched into lightpolarization planes of which are perpendicular to each other, outputsone thereof to the polarizer 23 j and output other thereof to thepolarizer 23 l.

Thereby, one branched light of branched light outputted from PBS 23 e islight constituted by multiplexing incident p polarized light (frequencyf1′) and emitted s polarized light (frequencies f1, f2), and otherbranched light becomes light multiplexed with incident p polarized light(frequency f1′) and emitted p polarized light (frequencies f1, f2).

Further, polarization planes of light multiplexed and branched by PBS 23e are perpendicular to each other and therefore, light multiplexed andbranched by PBS 23 e are interfered with each other by incliningpolarization planes by the polarizers 23 j, 23 l to be received by thelight receiving sections 16, 18.

Similarly, PBS 23 f synthesizes reference light (incident s polarizedlight (frequency f2′)) from the wave plate 23 h and signal light(emitted s polarized light (frequencies f1, f2), emitted polarized light(frequencies f1, f2)) to be thereafter branched to light polarizationplanes of which are perpendicular to each other, outputs one thereof tothe polarizer 23 i and outputs other thereof to the polarizer 23 k.

Thereby, one branched light of branch light outputted from PBS 23 f islight constituted by multiplexing incident s polarized light (frequencyf2′) and emitted s polarized light (frequencies f1, f2), and otherbranched light becomes light multiplexed with incident s polarized light(frequency f2′) and emitted p polarized light (frequencies f1, f2).

Further, polarization planes of light multiplexed and branched by PBS 23f are perpendicular to each other and therefore, light multiplexed andbranched by PBS 23 f are interfered with each other by incliningpolarization planes by the polarizers 23 i, 23 k to be received by thelight receiving sections 15, 17.

Output signals from the light receiving sections 15 through 18 arefiltered by respective the low-pass filters 19 through 22, respectivesignals after having been filtered become interference signals operatedonly by T₁₁, T₁₂, T₂₁, T₂₂ of Jones matrix. Further, operation otherthan the above-described is similar to that of the apparatus shown inFIG. 1 and therefore, an explanation thereof will be omitted.

Third Example of Interference Section

Next, FIG. 4 is a configuration diagram showing a third example of theinterference section of the apparatus shown in FIG. 1. Here, sectionsthe same as those of FIG. 1 are attached with the same notations and anexplanation thereof will be omitted. In FIG. 4, an interference section24 is provided in place of the interference section 14, the interferencesection 24 outputs interference light similar to that of theinterference section 14 and includes branching sections 24 a through 24c, multiplexing sections 24 d through 24 f, wave plates 24 g, 24 h, andPBS 24 i, 24 j.

Further, the branching sections 24 a through 24 c branch light withoutdepending on a polarized state, and are, for example, HM,non-polarization beam splitters, optical fiber couplers or the like. Themultiplexing sections 24 e, 24 f synthesize light without depending on apolarized state, and are, for example, HM, non-polarization beamsplitters, optical fiber couplers or the like. The wave plates 24 g, 24h are for example, ½ wave plates.

The first input light branching section 24 a branches incident ppolarized light (frequency f1) from the light source section 13 in two,outputs one thereof to the multiplexing section 24 d as signal light andoutputs other thereof to the wave plate 24 g as reference light.

The second input light branching section 24 b branches incident spolarized light (frequency f2) from the light source section 13 in two,outputs one thereof to the multiplexing section 24 d as signal light andoutputs other thereof to the wave plate 24 h as reference light.

The multiplexing section 24 d is, for example, PBS, a non-polarizationbeam splitter, HM, an optical fiber coupler or the like, synthesizessignal light from the branching sections 24 a, 24 b to be outputted tothe measuring object 1. The branching section 24 c branches output light(signal light) from the measuring object 1 in two, outputs one thereofto the multiplexing section 24 e and outputs other thereof to themultiplexing section 24 f.

The wave plate 24 g is provided between the branching section 24 a andthe multiplexing section 24 e for inclining a polarization plane ofincident p polarized light of reference light by 45°. The wave plate 24h is provided between the branching section 24 b and the multiplexingsection 24 f for inclining a polarization plane of emitted s polarizedlight of reference light by 45°.

The first output light multiplexing section 24 e synthesizes andinterferes one branched light (signal light) from the branching section24 c and incident p polarized light of reference light from the waveplate 24 g.

The second output light multiplexing section 24 f synthesizes andinterferes other branched light (signal light) from the branchingsection 24 c and incident s polarized light of reference light from thewave plate 24 h.

The first PBS 24 i branches multiplexed light from the multiplexingsection 24 e in two of light polarization planes of which areperpendicular to each other to be outputted to the light receivingsections 15, 18. The second PBS 24 j branches multiplexed light from themultiplexing section 24 f in two of light polarization planes of whichare perpendicular to each other to be outputted to the light receivingsections 15, 17.

Operation of the apparatus will be explained.

The branching section 24 a branches incident p polarized light(frequency f1) from the light source section 13 in two, outputs onethereof to the multiplexing section 14 d as signal light and outputsother thereof to the wave plate 24 g as reference light. Further, thebranching section 24 b branches incidents polarized light (frequency f2)from the light source section 13 in two, outputs one thereof to themultiplexing section 24 d as signal light, and outputs other thereof tothe wave plate 24 h as reference light. Further, respective the waveplates 24 g, 24 h incline polarization planes of reference light by 45°.

Further, the multiplexing section 24 d synthesizes signal light from thebranching sections 24 a, 24 b to be outputted to the measuring object 1.Further, when the multiplexing section 24 d synthesizes light by usingPBS, light can be multiplexed more efficiently than an optical element(for example, HM, non-polarization beam splitter, optical fiber coupleror the like) for multiplexing and branching light without depending on apolarized state.

Further, the branching section 24 c branches emitted s polarized light(frequencies f1, f2), emitted p polarized light (frequencies f1, f2)outputted from the measuring object 1 in two, outputs one thereof to themultiplexing section 24 e and outputs other thereof to the multiplexingsection 24 f. Naturally, emitted, s polarized light is operated by T₁₁,T₁₂, and emitted p polarized light is operated by T₂₁, T₂₂.

Further, the multiplexing section 24 e synthesizes and interferesreference light (incident p polarized light (frequency f1′)) from thewave plate 24 g and signal light (emitted s polarized light (frequenciesf1, f2)), emitted p polarized light (frequencies f1, f2)). Further, PBS24 i branches interference light multiplexed by the multiplexing section24 e to light polarization planes of which are perpendicular to eachother, outputs one thereof to the light receiving section 16 and outputsother thereof to the light receiving section 18.

Thereby, one branched light of branched light outputted from PBS 24 i isinterference light constituted by multiplexing incident p polarizedlight (frequency f1′) and emitted s polarized light (frequencies f1,f2), and other branched light becomes interference light multiplexedwith incident p polarized light (frequency f1′) and emitted p polarizedlight (frequencies f1, f2).

Further, respective branched light branched by PBS 24 i are received bythe light receiving sections 16, 18.

Similarly, the multiplexing section 24 f synthesizes and interferesreference light (incident s polarized light (frequency f2′)) from thewave plate 24 h and signal light (emitted s polarized light (frequenciesf1, f2), emitted p polarized light (frequencies f1, f2)). Further, PBS24 j branches interference light multiplexed by the multiplexing section23 f to light polarization planes of which are perpendicular to eachother, outputs one thereof to the light receiving section 15 and outputsother thereof to the light receiving section 17.

Thereby, one branched light of branched light outputted from PBS 24 j islight constituted by multiplexing incident s polarized light (frequencyf2′) and emitted s polarized light (frequencies f1, f2), and otherbranched light becomes interference light multiplexed with incident spolarized light (frequency f2′) and emitted p polarized light(frequencies f1, f2).

Further, respective branched light branched by PBS 24 j are received bythe light receiving sections 15, 17.

Further, output signals from the light receiving sections 15 through 18are filtered by respective the low-pass filters 19 through 22, andrespective signals after having been filtered become interferencesignals operated by only T₁₁, T₁₂, T₂₁, T₂₂ of Jones matrix. Further,operation other than the above-described is similar to that of theapparatus shown in FIG. 1 and therefore, an explanation thereof will beomitted.

Fourth Example of Interference Section

Next, FIG. 5 is a configuration diagram showing a fourth example of theinterference section in the apparatus shown in FIG. 1. Here, sectionsthe same as those of FIG. 1, FIG. 4 are attached with the same notationsand an explanation thereof will be omitted. In FIG. 5, HM 24 k isconstituted by commonly integrating the branching sections 24 a, 24 b,HM 24 l is constituted by commonly integrating the multiplexing sections24 e, 24 f, PBS 24 m is constituted by commonly integrating PBS 24 i, 24j.

The light source section 14 includes a wavelength variable light source13 a, optical fibers 13 b, 13 c, lenses (collimator section) 13 d, 13 e,a polarized wave controller 13 f. The wavelength variable light source13 a subjects respective light having different frequencies towavelength sweep to be outputted. The optical fiber 13 b transmits lightof the frequency f1 from the light source section 13 a. The opticalfiber 13 c transmits light of the frequency f2 from the light sourcesection 13 a. Further, the optical fibers 13 b, 13 c are installed suchthat optical axes of emitted light become in parallel with each other.

Respective the lenses 13 d, 13 e make light emitted from the opticalfibers 13 b, 13 c parallel light. The polarized wave controller 13 f,for example, arranged in series with ¼ wave plates, ½ wave plates isprovided to optical paths, converts light from the lens 13 d intoincident p polarized light (first input light) and converts light fromthe lens 13 e into incident s polarized light (second input light) to beoutputted to the interference section 24.

A mirror 24 n reflects incident s polarized light from HM 24 k to PBS 24d.

A lens (light converging section) 24 o makes light multiplexed by PBS 24d incident on an optical fiber 24 p. The optical fiber 24 p transmitsincident p polarized light, incident s polarized light from theinterference section 24 to the measuring object 1.

An optical fiber 24 q transmits emitted p polarized light, emitted spolarized light from the measuring object 1. A lens (collimator section)24 r makes emitted p polarized light, emitted s polarized light from theoptical fiber 24 q parallel light to be outputted to HM 24 c. A mirror24 s reflects one branched light from HM 24 c to HM 24 l.

Respective polarizers 24 t through 24 w are provided between PBS 24 mand the light receiving section 16, between PBS 24 m and the lightreceiving section 17, between PBS 24 m and the light receiving section18, between PBS 24 m and the light receiving section 19 to pass light ofonly a predetermined polarization plane.

Operation of the apparatus will be explained.

Respective light of the frequencies f1, f2 from the wavelength variablelight source 13 a are transmitted to the polarized wave controller 13 fby the fibers 13 b, 13 c, the lenses 13 d, 13 e. Further, the polarizedwave controller 13 f converts respective polarized light states of lightof the frequencies f1, f2 to be outputted to HM 24 k of the interferencesection 24 as incident p polarized light, incident s polarized light.

Further, HM 24 k branches incident s polarized light, incident ppolarized light to signal light and reference light. Branched incident spolarized light (signal light) is reflected by the mirror 24 n,multiplexed with incident p polarized light (signal light) by PBS 24 d,and is inputted to the measuring object 1 by way of the lens 24 o, theoptical fiber 24 p.

Further, emitted p polarized light, emitted s polarized light from themeasuring object 1 are inputted to HM 24 c by way of the optical fiber24 q, the lens 24 r.

Further, HM 24 c branches emitted p polarized light, emitted s polarizedlight to two, one branched light is reflected by the mirror 24 s and isincident on HM 24 l, other branched light is incident on HM 24 l.

Thereby, HM 24 l synthesizes reference light (incident p polarized light(frequency f1′)) from the wave plate 24 g, signal light (emitted spolarized light (frequencies f1, f2), emitted p polarized light(frequencies f1, f2)) and synthesizes reference light (incident spolarized light (frequency f2′)) from the wave plate 24 h, signal light(emitted s polarized light (frequencies f1, f2), emitted p polarizedlight (frequencies f1, f2)).

Further, PBS 24 m branches interference light multiplexed by themultiplexing section 24 l to light polarization planes of which areperpendicular to each other to be outputted to the light receivingsections 15 through 18 by way of the polarizers 24 t to 24 w. Here,different from the polarizers shown in FIG. 2, FIG. 3, the polarizers 24t through 24 w pass light of only a predetermined polarization plane.For example, the polarizers 24 t, 24 u pass light of a polarizationplane the same as that of incident s polarized light from the polarizedwave controller 13 f, the polarizers 24 b, 24 w pass light of apolarization plane the same as that of incident p polarized light fromthe polarized wave controller 13 f. That is, this is because it isdifficult for PBS 24 m to completely branch inputted light to lightpolarization plane of which are perpendicular to each other and lightwhich has not been branched is removed.

Further, the light receiving sections 15 through 18 receive interferencesignals to be outputted to the low-pass filters 19 through 22, notillustrated, at a rear stage. Further, outputted signals from the lightreceiving sections 15 through 18 are filtered by respective the low-passfilters 19 through 22. Thereby, respective signals after having beenfiltered become interference signals operated only by T₁₁, T₁₂, T₂₁, T₂₂of Jones matrix. Further, operation other than the above-described issimilar to that of the apparatus shown in FIG. 1 and therefore, anexplanation thereof will be omitted.

In this way, the polarizers 24 t through 24 w pass only light of apredetermined polarization plane to be outputted to the light receivingsections 15 through 18 and therefore, noise of the interference signalscan be reduced.

Further, the branching sections 24 a, 24 b are made common, themultiplexing sections 24 e, 24 f are made common, PBS 24 i, 24 j aremade common and therefore, a number of parts is reduced to achievesmall-sized formation, facilitating alignment, a reduction in cost.

Further, by constituting the interference section by an interferometerof a spatial light type, an optical system can be downsized and is madeto be strong at vibration.

Further, the invention is not limited to the first embodiment and thefirst to fourth examples of the interference section but may be as shownbelow.

Although there is shown a configuration in which the light sourcesection 13 outputs p polarized light, s polarized light which arelinearly polarized light and polarization planes of which areperpendicular to each other as first, second input light in theapparatus shown in FIG. 1 through FIG. 5, first, second input light maybe constituted by polarized light polarized states of which areperpendicular to each other (for example, circularly polarized light,elliptically polarized light).

Although there is shown a configuration in which output light (emitted ppolarized light, emitted s polarized light) for interfering withreference light are respectively branched to linearly polarized light inthe apparatus shown in FIG. 1 through FIG. 5, output light includingfrequencies f1, f2 may be branched to the light of a first polarizedstate, a second polarized state, respective which may interfere withreference light. Further, first, second polarized states areperpendicular to each other.

Although there is shown a configuration of providing the wave plates 14f, 14 g, 23 g, 23 h, 24 g, 24 h in the apparatus shown in FIG. 2 throughFIG. 5, the wave plates may not be provided when a polarization plane oflight inputted to PBS at a rear stage is inclined to an optical axis ofPBS.

Although there is shown a configuration of using an interferometer ofMach-Zender type for the interference sections 14, 23, 24 in theapparatus shown in FIG. 2 through FIG. 5, any two light fluxinterferometer may be used.

In the apparatus shown in FIG. 3, similar to the apparatus shown in FIG.5, the branching sections 23 a, 23 b may be made common, PBS 23 e, 23 fmay be made common.

Although there is shown a configuration of providing the polarized wavecontroller 13 f between HM 24 k and the lenses 13 d, 13 e in theapparatus shown in FIG. 5, for example, the polarized wave controllers13 f may be provided at middles of the optical fibers 13 b, 13 c, lightemitted from the optical fibers 13 b, 13 c may already become incident ppolarized light, incident s polarized light.

Although there is shown a configuration of providing the polarizers 24 tthrough 24 w in the apparatus shown in FIG. 5, the polarizers 24 tthrough 24 w may not be provided.

Second Embodiment

FIG. 6 is a configuration diagram showing a second embodiment of theinvention. Here, sections the same as those of FIG. 14, FIG. 16 areattached with the same notations and an explanation thereof will beomitted. In FIG. 6, a light source section LS1 is provided in place ofthe wavelength variable light source 2. The light source section LS1includes a first wavelength variable light source 122, a secondwavelength variable light source 132, a detecting section 142, a controlsection 152, and outputs first, second input light having apredetermined frequency difference therebetween to an interferencesection 100.

The first wavelength variable light source 122 on a master side includesan LD light source 122 a, a wavelength sweep circuit 122 b forsubjecting p polarized light (first input light) to wavelength sweep tobe outputted to the interference section 100. The LD light source 122 asubjects a measuring wavelength range continuously to wavelength sweepto output laser light by an instruction from the wavelength sweepcircuit 122 b.

The second wavelength variable light source 132 on a slave side includesan LD light source 132 a, a wavelength sweep circuit 132 b forsubjecting s polarized light (second input light) to wavelength sweep tobe outputted to the interference section 100. The LD light source 132 asubjects a measuring wavelength range continuously to wavelength sweepto output laser light by an instruction from the wavelength sweepcircuit 132 b.

The LD light sources 122 a, 132 a for outputting laser light are surfaceemitting lasers forming resonators by movable mirrors (reflectinglayers) formed by a semiconductor micromachining technology. Further,the surface emitting laser (VCSEL: Vertical-Cavity Surface-EmittingLaser) is constituted by a structure of interposing a semiconductorlayer by a reflecting layer formed by multilayered films or the like.Further, the semiconductor layer is formed by multilayers including anactive layer and a spacer layer (referred to also as clad layer) forinterposing the active layer (refer to, for example, “Connie J.Chang-Hasnain, “tunable VCSEL”, by IEEE JOURNAL ON SELECTED TOPICS INQUANTUM ELECTRONICS, Vol. 6, No. 6, NOVEMBER/DECEMBER 2000, pp 978-987”or “D. Vakhashoori, P. D. Wang, M. Azimi, K. J. Knopp, M. Jiang,“MEMs-Tunable Vertical-Cavity Surface-Emitting Lasers”, Proc. ofOFC2001, TuJ1-1 through TuJ1-3” or the like.

The detecting section 142 includes a polarizer 142 a, a light receivingsection 142 b, and detects a frequency difference of s polarized light,p polarized light outputted by the wavelength variable light sources122, 132. The polarizer 142 a rotates polarization planes of s polarizedlight, p polarized light to be interfered with each other. The lightreceiving section 142 b receives interference light of s polarizedlight, p polarized light and outputs a signal in accordance withreceived optical power.

The control section 152 controls a wavelength sweep speed of thewavelength variable light source 132 based on the frequency differencedetected by the detecting section 142 and controls constant thefrequency difference of laser light outputted by the plane variablelight sources 122, 132.

At the interference section 100, the polarized light delay section 6 cis removed and PBS 162, HM 172 are provided between HM 3 and themeasuring object 1. Further, a polarization plane rotating section 182is provided between HM 3 and HM 7 (optical path on reference lightside).

PBS 162 multiplexes one branched light from HM 3 and s polarized lightfrom the wavelength variable light source 132. HM 172 branches lightfrom PBS 162 and outputs one thereof to the measuring object 1 andoutputs other thereof to the detecting section 142.

The polarization plane rotating section 182 is a ½ wave plate, when, forexample, an interval between HM 3 and HM 7 is constituted by spatiallight, an incident end and an emitting end thereof are installed to beinclined by 45° when the interval is constituted by a polarized waveholding optical fiber for rotating a polarization plane of otherbranched light from HM 3 by 45° relative to an optical axis of PBS 8.

Operation of the apparatus will be explained.

Respective the wavelength sweep circuits 122 b, 132 b of the wavelengthvariable light sources 122, 132 make the LD light sources 122 a, 132 aoutput laser light of frequencies f1, f2 to be subjected to wavelengthsweep by a predetermined wavelength sweep speed. Further, the wavelengthsweep circuits 122 b, 132 b read set values (start wavelength/finishwavelength, sweep speed and the like of wavelength sweep) from a memory,not illustrated, and issue an instruction to the LD light sources 122 a,132 a in accordance with the set values. Further, a set value of afrequency difference (|f1−f2|) of the wavelength variable light sources122, 132 in starting to output laser light is set to, for example, 50[MHz].

Further, a polarized wave controller, not illustrated, convertspolarized states of laser light respectively from the wavelengthvariable light sources 122, 132 top polarized light, s polarized lightto output to the interference section 100 as incident p polarized light,incident s polarized light. Naturally, when laser light outputted fromthe wavelength variable light sources 122, 132 are constituted by ppolarized light, s polarized light, the polarized wave controller is notneeded.

Further, HM 3 of the interference section 100 branches incident ppolarized light, outputs one thereof to PBS 162 as signal light andoutputs other thereof to the polarization plane rotating section 182 asreference light. Further, the polarization plane rotating section 182inclines a polarization plane of reference light by 45° such thatoptical power is uniformly branched at PBS 8 at a rear stage.

Successively, an explanation will be given of a side of one branchedlight (signal light) from HM 3. PBS 162 multiplexes incident s polarizedlight and incident p polarized light from HM 3. Naturally, sincemultiplexed by PBS 162, incident s polarized light and incident ppolarized light become linearly polarized light perpendicular to eachother. Further, HM 172 branches multiplexed light from PBS 162 to outputone thereof to the measuring object 1 and outputs other thereof to thedetecting section 142.

Further, in order to make incident p polarized light and incident spolarized light interfere with each other, the polarizer 142 a of thedetecting section 142 inclines a polarization plane to thereby makeincident p polarized light and incident s polarized light interfere witheach other to be outputted to the light receiving section 142 b.Further, the light receiving section 142 b outputs a signal inaccordance with optical power of interference light, the detectingsection 142 obtains a frequency of an interference signal (beat signal)from the light receiving section 142 b and detects a frequencydifference of incident p polarized light and incident s polarized lightto be outputted to the control section 152.

Further, the control section 152 controls the wavelength sweep circuit132 b of the wavelength variable light source 132 based on the frequencydifference detected by the detecting section 142 to thereby make thefrequency difference of laser light outputted by the wavelength variablelight sources 122, 132 constant (50 [MHz]). Further, the control section152 reads the value of the frequency difference previously from amemory, not illustrated.

Further, operation thereafter of multiplexing reference light from thepolarization plane rotating section 182 and output light from themeasuring object 1 by HM7 to be outputted to PBS 8 to be received by thelight receiving sections 9, 10 is similar to that of the apparatus shownin FIG. 16.

That is, interference light branched by PBS 8 and inputted to the lightreceiving section 9 is by output light combined with T₁₁, T₁₂ of Jonesmatrix and reference light. Further, interference light inputted to thelight receiving section 10 is constituted by output light combined withT₂₁, T₂₂ of Jones matrix and the reference light.

Further, an interference signal influenced by T₁₁ is incident spolarized light (frequency f2) outputted from the wavelength variablelight source 132 on the slave side and passed through the measuringobject 1. That is, a frequency thereof differs from that of referencelight (frequency f1′) from the wavelength variable light source 122 onthe master side by about 50 [MHz]). Therefore, the interference signalof emitted s polarized light (frequency f2) influenced by T₁₁ and spolarized light (frequency f1′) of reference light is provided at avicinity of the frequency of 50 [MHz]. On the other hand, theinterference signal of emitted s polarized light (frequency f1)influenced by T₁₂ and s polarized light (frequency f1′) of referencelight is provided at a vicinity of DC.

By utilizing the frequency difference, from interference signalsoutputted from the light receiving section 9, the filter circuit 101extracts an interference signal (high frequency component) of emitted spolarized light of T11 and reference light by a band-pass filter(passing band, vicinity of 50 [MHz]), extracts the interference signal(low frequency component) of emitted s polarized light of T₁₂ andreference light by a low-pass filter (passing band, vicinity of DC), andoutputs the respectively filtered interference signals to thecalculating section 103.

Similarly, also with regard to interference signals influenced by T₂₁,T₂₂ provided by the light receiving section 10, by utilizing thefrequency difference, from the interference signals outputted from thelight receiving section 10, the filter circuit 102 extracts aninterference signal (high frequency component) of emitted p polarizedlight of T₂₁ and reference light (p polarized light) by a band-passfilter (passing band, vicinity of 50 [MHz]), extracts an interferencesignal (low frequency component) of emitted p polarized light of T₂₂ andreference light (p polarized light) by a low-pass filter (passing band,vicinity of DC), and outputs the respectively filtered interferencesignals to the calculating section 103.

Further, the calculating section 103 obtains respective elements ofJones matrix from amplitudes and phases of 4 pieces of the interferencesignals filtered by the filter circuits 101, 102 and obtains an opticalcharacteristic of the measuring object 1 from Jones matrix.

In this way, the detecting section 142 detects the frequency differenceof light outputted from the wavelength variable light sources 122, 132,the control section 152 controls the wavelength sweep speed of thewavelength variable light source 132 based on the frequency differencedetected by the detecting section 142 and therefore, the wavelengthvariable light source 132 on the slave side is subjected to wavelengthsweep while maintaining the constant light frequency difference(|f1−f2|) relative to the wavelength variable light source 122 on themaster side. Thereby, a center frequency (|f1−f2|) of the interferencesignals from the light receiving sections 9, 10 is not varied.Therefore, even when the wavelength sweep speed of the wavelengthvariable light source 122 on the master side is not constant, Jonesmatrix of the measuring object 1 can accurately be measured.

Further, PBS 162 of the interference section 100 multiplexes input lightof p polarized light, s polarized light to be outputted to the measuringobject 1 and therefore, first, second input light can be multiplexedmore efficiently than those in the case of using HM. Thereby, loss ofoptical power can be restrained and interference light having strongoptical power can be provided.

Further, LD light source 122 a, 132 a of the wavelength variable lightsources 122, 132 output laser light by using surface emitting lasersforming oscillators by movable mirrors formed by a semiconductormicromachining technology and therefore, cost can be reduced, andwavelength sweep speed can be accelerated. Thereby, a number of times ofwavelength sweep within a predetermined time period is increased, anaveraging processing can be increased, and accuracy of measurement ispromoted. Further, although the interferometer of the interferencesection 100 is much liable to be effected with an influence ofdisturbance (vibration), by shortening wavelength sweep time, theinfluence of the disturbance can be restrained, and accuracy ofmeasurement is promoted.

Third Embodiment

FIG. 7 is a configuration diagram showing a third embodiment of theinvention. Here, sections the same as those of FIG. 6 are attached withthe same notations and an explanation thereof will be omitted. In FIG.7, a light source section LS2 is provided in place of the light sourcesection LS1. The light source section LS2 is provided with thewavelength variable light source 122, HM 192, an acousto-opticalmodulator (hereinafter, abbreviated as AOM) 202, polarized wavecontrollers 212, 222. Further, HM 172 of the interference section 100 isremoved.

HM 192 is a branching section which branches laser light from thewavelength variable light source 122 for outputting laser light bycarrying out wavelength sweep, outputs one thereof to the polarized wavecontroller 212, and outputs other to AOM 202. AOM 202 shifts a frequencyof other branched light from HM 192 by the predetermined amount, forexample, 50 [MHz].

The polarized wave controller 212 converts one branched light from HM192 to p polarized light (first input light) to be outputted to HM 3 ofthe interference section 100. The polarized wave controller 222 convertslight from AOM 202 to s polarized light (second input light) to beoutputted to PBS 162 of the interference section 100.

Operation of the apparatus will be explained.

The wavelength sweep circuit 122 b of the wavelength variable lightsource 122 makes the LD light source 122 a output laser light similar tothe apparatus shown in FIG. 6 to be subjected to wavelength sweep by apredetermined wavelength sweep speed. Further, the wavelength sweepcircuit 122 b reads set values (start wavelength, finish wavelength,sweep speed and the like of wavelength sweep) from a memory, notillustrated, and issues an instruction to the LD light source 122 a inaccordance with the set values.

Further, HM 192 branches laser light from the wavelength variable lightsource 122 in two, outputs one thereof to the polarized wave controller212 and outputs other thereof to AOM 202. Further, the polarized wavecontroller 212 converts one branched light branched light HM 192 into ppolarized light to be outputted to HM 3 of the interference section 100.

Further, AOM 202 shifts a frequency of other branched light branched byHM 192 by 50 [MHz] to be thereafter outputted to the polarized wavecontroller 222. Further, the polarized wave controller 222 converts thelight from AOM 202 to s polarized light to be outputted to PBS 162 ofthe interference section 100. Therefore, incident p polarized light andincident s polarized light inputted to the interference section 100 areprovided with the frequency difference of 50 [MHz]. Further, the otheroperation is similar to that of the apparatus shown in FIG. 6 andtherefore, an explanation thereof will be omitted.

In this way, AOM 202 shifts laser light outputted from the wavelengthvariable light source 122 by the predetermined amount (50 [MHz]) to beoutputted to the interference section 100 and therefore, lightmaintaining the constant light frequency difference (|f1−f2|) relativeto the wavelength variable light source 122 is outputted. Thereby, thecenter frequency (|f1−f2|) of the interference signals from the lightreceiving sections 9, 10 is not varied. Therefore, even when thewavelength sweep speed of the wavelength variable light source 12 is notconstant, Jones matrix of the measuring object 1 can accurately bemeasured.

Fourth Embodiment

FIG. 8 is a configuration diagram showing a fourth embodiment of theinvention. Here, sections the same as those of FIG. 6 are attached withthe same notations and an explanation thereof will be omitted. In FIG.8, HM 7 of the interference section 100 is removed and PBS 8 multiplexesand branches reference light and signal light. Further, a polarizer 232is provided between PBS 8 and the light receiving section 9 and apolarizer 242 is provided between PBS 8 and the light receiving section10.

Operation of the apparatus will be explained.

PBS 8 branches output light to linearly polarized light perpendicular toeach other, further, also branches reference light to linearly polarizedlight perpendicular to each other, multiplexes respective branchedoutput light and reference light to be outputted to the polarizers 232,242. Polarization planes of multiplexed light are perpendicular to eachother and are not interfered with each other and therefore, thepolarizers 232, 242 incline the polarization planes to be interferedwith each other to be outputted to the light receiving sections 9, 10.The other operation is similar to that of the apparatus shown in FIG. 6and therefore, an explanation thereof will be omitted.

In this way, PBS 8 carries out multiplexing and branching and therefore,in comparison of the case of using HM 7, the interference section 100can be downsized and the optical system can be facilitated to beadjusted.

Fifth Embodiment

Although in the apparatus shown in FIG. 6 through FIG. 8, in order toobtain respective elements of T₁₁ through T₂₂ of Jones matrix, there areshown configurations of separating the low frequency components and thehigh frequency components from the interference signals of the lightreceiving sections 9, 10 by the filter circuits 101, 102, theinterference section may be constituted such that all of theinterference signals for obtaining the respective elements are providedby the low frequency components at vicinities of DC.

That is, output light from the measuring object 1 includes emitted spolarized light (frequencies f1, f2), emitted p polarized light(frequencies f1, f2), and the interference section is constituted tooutput interference light of combinations (a) through (d) shown below.

(a) p polarized light (frequency f1′) of reference light and emitted ppolarized light (frequencies f1, f2) of signal light.

(b) s polarized light (frequency f2′) of reference light and emitted ppolarized light (frequencies f1, f2) of signal light.

(c) p polarized light (frequency f1′) of reference light and emitted spolarized light (frequencies f1, f2) of signal light.

(d) s polarized light (frequency f2′) of reference light and emitted spolarized light (frequencies f1, f2) of signal light.

By receiving interference light of (a) through (d) described above bythe light receiving section to be filtered by the low-pass filter at arear stage, all of interference signals for obtaining the respectiveelements can be provided by low frequency components at vicinities ofDC.

Here, similar to FIG. 6 through FIG. 8, signal light and reference lightat the interference section are transmitted by the different opticalpaths and multiplexed and therefore, the frequencies f1′, f2′ ofreference light are produced by the optical length difference.Therefore, when the frequencies of reference light immediately beforebeing multiplexed with signal light are designated by notations f1′,f2′, the frequency difference (|f1−f2|) of first, second input lightoutputted by the light source section LS1 is set to be sufficientlylarger than frequency difference (|f2−f2′|), (|f1−f1′|) produced by theoptical length difference of signal light and reference light.

FIG. 9 is a configuration diagram showing a fifth embodiment of theinvention. Here, sections the same as those of FIG. 8 are attached withthe same notations and an explanation thereof will be omitted. In FIG.9, an interference section 200 is provided in place of the interferencesection 100.

The interference section 200 outputs interference light of (a) through(d) mentioned above. Further, multiplexed light of incident s polarizedlight and incident p polarized light is outputted from the lightreceiving section LS1.

The light receiving sections 38 through 41 are provided in place of thelight receiving sections 9, 10, provided for respective interferencelight outputted from the interference section 200, receives interferencelight and outputs signals in accordance with optical power ofinterference light.

The low-pass filters 42 through 45 are provided in place of the filtercircuits 101, 102, provided for respective the light receiving sections38 through 41, filter signals outputted from the respective lightreceiving sections 38 through 41, pass only signals of frequencycomponents lower than the frequency difference (|f1−f2|) of incident spolarized light and incident p polarized light to be outputted to thecalculating section 103 (not illustrated).

Operation of the apparatus will be explained.

The light source section LS1 outputs incident p polarized light,incident s polarized light subjected to wavelength sweep continuously inpredetermined wavelength ranges. Naturally, the light source section LS1controls incident p polarized light and incident s polarized light suchthat a frequency difference therebetween becomes constant based onmultiplexed light from the interference section 200.

The interference section 200 branches incident s polarized light,incident p polarized light and outputs one thereof to the measuringobject 1 as signal light. Naturally, output light from the measuringobject 1 includes emitted s polarized light and emitted p polarizedlight in correspondence with incident s polarized light, and emitted spolarized light and emitted p polarized light in correspondence withincident p polarized light.

Further, the interference section 200 multiplexes emitted s polarizedlight, emitted p polarized light from the measuring object 1 withincident s polarized light, incident p polarized light of referencelight to be interfered with each other.

Specifically, interference light of emitted s polarized light(frequencies f1, f2) and incident s polarized light (f2′) is outputtedto the light receiving section 38, interference light of emitted spolarized light (frequencies f1, f2) and incident p polarized light(f1′) is outputted to the light receiving section 39, interference lightof emitted p polarized light (frequencies f1, f2) and incident spolarized light (f2′) is outputted to the light receiving section 40,interference light of emitted p polarized light (frequencies f1, f2) andincident p polarized light (f1′) is outputted to the light receivingsection 41.

Further, the respective light receiving sections 38 through 41 outputsignals in accordance with optical power of interference light to thelow-pass filters 42 through 45. Further, the low-pass filters 42 through45 pass signals of low frequency components (for example, DC throughabout 200 [kHz]) of interference signals outputted from the lightreceiving sections 38 through 41 to be outputted to the calculatingsection 103, not illustrated, at a rear stage.

A specific explanation will be given by the light receiving section 38.The light receiving section 38 is inputted with emitted s polarizedlight (frequencies f1, f2), that is, signal light operated by T₁₁, T₁₂of Jones matrix and reference light (frequency f2′). Therefore, byfiltering the interference signal of the light receiving section 38 bythe low-pass filter 42, as the interference signal (incident s polarizedlight of frequency f2′, emitted s polarized light of frequency f2) afterhaving been filtered, only the interference signal operated by only T₁₁of Jones matrix is extracted.

Generally, as the interference signals after having been filtered byrespective the low-pass filters 42 through 45, only the interferencesignals operated by only T₁₂, T₂₁, T₂₂ of Jones matrix are extracted.

Further, the calculating section 103 obtains the respective elements ofJones matrix from amplitudes and phases of the interference signalsconstituting output signals from the low-pass filters 42 through 45 andobtains an optical characteristic of the measuring object 1 from Jonesmatrix.

Next, details of the interference section 200 will be explained.

The interference section 200 includes branching sections 252 through282, a multiplexing section 292, PBS 30, 31, wave plates 32, 33,polarizers 34 through 37.

Further, the branching sections 252 through 282 branch light withoutdepending on the polarized state, and are, for example, HM,non-polarization beam splitters, optical fiber couplers or the like.Further, the wave plates 32, 33 are polarization plane rotating sectionsand are, for example, ½ wave plates or the like.

The first input light branching section 252 branches incident ppolarized light (frequency f1) from the light source section LS1 in two,outputs one thereof to the multiplexing section 292 as signal light andoutputs other thereof to the wave plate 32 as reference light.

The second input light branching section 262 branches incident spolarized light (frequency f2) from the light source section LS1 in two,outputs one thereof to the multiplexing section 292 as signal light andoutputs other thereof to the wave plate 33 as reference light.

The multiplexing section 292 is, for example, PBS, a non-polarizationbeam splitter, HM, an optical fiber coupler or the like for multiplexingsignal from the light branching sections 252, 262 to be outputted to themeasuring object 1.

The branching section 272 for the light source is provided between themultiplexing section 292 and the measuring object 1 for branchingmultiplexed light of incident p polarized light, incident s polarizedlight to be outputted to the measuring object 1, the light sourcesection LS1.

The output light branching section 282 branches output light (signallight) from the measuring object 1 in two, outputs one thereof to PBS 30and outputs other thereof to PBS 31.

The wave plate 32 is provided between the branching section 252 and PBS30 and inclines a polarization plane of incident p polarized light ofreference light by 45°. The wave plate 33 is provided between thebranching section 262 and PBS 31 and inclines a polarization plane ofincident s polarized light of reference light by 45°.

The first PBS 30 multiplexes one branched light (signal light) from thebranching section 282 and incident p polarized light of reference lightfrom the wave plate 32 to be branched in two of light of polarizationplanes perpendicular to each other to be outputted to the polarizers 35,37.

The second PBS 31 multiplexes other branched light (signal light) fromthe branching section 282 and incident s polarized light of referencelight from the wave plate 33 to be branched in two of light ofpolarization planes perpendicular to each other to be outputted to thepolarizers 34, 36.

Respective the polarizers 34 through 37 are provided for respectivebranched light of PBS 30, 31, that is, provided between PBS 31 and thelight receiving section 38, between PBS 30 and the light receivingsection 39, between PBS 31 and the light receiving section 40, betweenPBS 30 and the light receiving section 41.

Operation of the interference section 200 will be explained.

The branching section 252 branches incident p polarized light (frequencyf1) from the light source section LS1 in two, outputs one thereof to themultiplexing section 292 as signal light and outputs other thereof tothe wave plate 32 as reference light. Further, the branching section 262branches incidents polarized light (frequency f2) from the light sourcesection LS1 in two, outputs one thereof to the multiplexing section 292as signal light and outputs other thereof to the wave plate 33 asreference light. Further, respective the wave plates 32, 33 inclinepolarization planes of reference light by 45°.

Further, the multiplexing section 292 multiplexes signal light from thebranching sections 252, 262 to be outputted to the branching section272. The branching section 272 branches multiplexed light to beoutputted to the measuring object 1, the light source section LS1.Further, when the multiplexing section 292 multiplexes light by usingPBS, PBS can multiplexes p polarized light, s polarized light moreefficiently than an optical element (for example, HM, a non-polarizationbeam splitter, an optical fiber coupler or the like) for multiplexingand branching light without depending on the polarized state. Thereby,loss of optical power can be restrained and interference light havingstrong optical power can be provided.

Further, the branching section 282 branches output light (emitted ppolarized light (frequencies f1, f2), emitted s polarized light(frequencies f1, f2)) outputted from the measuring object 1 in two,outputs one thereof to PBS 30 and outputs other thereof to PBS 31.Naturally, emitted s polarized light is operated by T₁₁, T₁₂, emitted ppolarized light is operated to T₂₁, T₂₂.

Further, PBS 30 multiplexes reference light (incident p polarized light(frequency f1′)) from the wave plate 32, signal light (emitted spolarized light (frequencies f1, f2), emitted p polarized light(frequencies f1, f2)) to be thereafter branched to light polarizationplanes of which are perpendicular to each other, outputs one thereof tothe polarizers 35 and outputs other thereof the to the polarizer 37.

Thereby, one branched light of branched light outputted from PBS 30 islight multiplexed with incident p polarized light (frequency f1′) andemitted s polarized light (frequencies f1, f2), and other branched lightbecomes light multiplexed with incident p polarized light (frequencyf1′) and emitted p polarized light (frequencies f1, f2).

Further, polarization planes of light multiplexed and branched by PBS 30are perpendicular to each other and therefore, interfered with eachother by inclining polarization planes by the polarizers 35, 37 to bereceived by the light receiving sections 39, 41.

Similarly, PBS 31 multiplexes reference light (incident s polarizedlight (frequency f2′)) from the wave plate 33 and signal light (emitteds polarized light) frequencies f1, f2), emitted p polarized light(frequencies f1, f2)) to be thereafter branched to light polarizationplanes of which are perpendicular to each other, outputs one thereof tothe polarizer 34 and outputs other thereof to the polarizer 36.

Thereby, one branched light of branched light outputted from PBS 31 islight multiplexed with incident s polarized light (frequency f2′) andemitted s polarized light (frequencies f1, f2) and other branched lightbecomes light multiplexed with incident s polarized light (frequencyf2′) and emitted p polarized light (frequencies f1, f2).

Further, polarization planes of light multiplexed and branched by PBS 31are perpendicular to each other and therefore, interfered with eachother by inclining polarization planes by the polarizers 34, 36 to bereceived by the light receiving sections 38, 40.

Further, respective the low-pass filters 42 through 45 filter outputsignals from the light receiving sections 38 through 40 as describedabove, respective signals after having been filtered becomesinterference signals operated by only T₁₁, T₁₂, T₂₁, T₂₂ of Jonesmatrix.

In this way, the interference section 200 outputs interference light ofincident p polarized light and emitted s polarized light, interferencelight of incident p polarized light and emitted p polarized light,interference light of incident s polarized light and emitted s polarizedlight, interference light of incident s polarized light and emitted ppolarized light and filters the interference signals by the low-passfilters 42 through 45. Thereby, the interference signals of lowfrequency components passing through the low-pass filters 42 through 25can further alleviate an influence of the frequency difference ofincident s polarized light and incident p polarized light produced bynonlinearity of frequency sweep of the light source section LS1.Therefore, even when the frequency difference of incident p polarizedlight and incident s polarized light is varied, the frequency differencecan accurately be measured. Further, only the signals of the lowfrequency components are dealt with and therefore, a band-pass filter isnot needed, circuit design of the low-pass filters 42 through 45,electric circuits or the like at a rear stage of the filters isfacilitated and circuit configuration is simplified.

Further, the invention is not limited to the second to the fifthembodiments but may be as shown below.

Although in the apparatus shown in FIG. 6 through FIG. 8, there is showna configuration of branching light by HM 3, 172, 192, any configurationwill do so far as the configuration branches light without depending onthe polarized state and may be, for example, a non-polarization beamsplitter, an optical fiber coupler or the like.

Although in the apparatus shown in FIG. 6 through FIG. 8, there is showna configuration of constituting reference light by branching incident ppolarized light, reference light may be constituted by branchingincident s polarized light, and both of incident p polarized light,incident s polarized light may be used for reference light.

In the apparatus shown in FIG. 6, FIG. 8, VCSEL 122 a, 132 a may beprovided on the same board and may be formed by one chip.

Although in the apparatus shown in FIG. 6, FIG. 8, FIG. 9, there isshown a configuration of controlling the wavelength sweep speed of thewavelength variable light source 132 by the control section 152, awavelength sweep speed of the wavelength variable light source 122 orboth of the wavelength variable light sources 122, 132 may becontrolled.

Although in the apparatus shown in FIG. 6 through FIG. 9, there is showna configuration in which the light sources section LS1, LS2 outputfirst, second input light by p polarized light, s polarized light whichare constituted by linearly polarized light and polarization planes ofwhich are perpendicular to each other, the polarized states of first,second input light may be perpendicular to each other, and first, secondinput light may be constituted by, for example, circularly polarizedlight, elliptically polarized light or the like.

Although in the apparatus shown in FIG. 6 through FIG. 9, there is showna configuration of branching output light (emitted p polarized light,emitted s polarized light) for interfering with reference lightrespectively to linearly polarized light, output light includingfrequencies f1, f2 may be branched to light of a first polarized stateand light of a second polarized state to be respectively interfered withreference light. Further, the first, the second polarized states areperpendicular to each other.

In the apparatus shown in FIG. 6 through FIG. 9, the interferencesection 100, 200 may be constituted by an interferometer of a spatiallight type. By constituting the interference section 100, 200 by theinterferometer of the spatial light type, an optical system can bedownsized and can be made to be strong at vibration.

Although in the apparatus shown in FIG. 6 through FIG. 9, there is showna configuration of using the interferometer of Mach-Zender type of theinterference sections 100, 200, any two light flux interferometer may beused.

Although in the apparatus shown in FIG. 6 through FIG. 9, there is showna configuration of using VCSEL for the LD light sources 122 a, 132 a,other wavelength variable laser may be used.

In the apparatus shown in FIG. 7, as shown by FIG. 8, HM 7 of theinterference section 100 may be removed and signal light and referencelight may be multiplexed and branched by PBS 8.

In the apparatus shown in FIG. 7, as shown by FIG. 9, the interferencesection 200 may be used in place of the interference section 100. Inthis case, the branching section 272 may not be provided.

Sixth Embodiment

FIG. 10 is a configuration diagram showing a sixth embodiment of theinvention. FIG. 11 is a diagram showing in details of an essentialsection of the apparatus shown in FIG. 10. Here, sections the same asthose of FIG. 17 are attached with the same notations and an explanationthereof will be omitted. In FIG. 10, an interference section 1066 isprovided in place of the interference section 666. Further, photodiodearrays 1166, 1266 are provided in place of the photodiodes 766, 866.Further, interference signal converting sections 1366, 1466 are providedbetween the photodiode arrays 1166, 1266 and the calculating section966.

The interference section 1066 includes HM 1066 a, mirrors 1066 b, 1066c, PBS 1066 d, a polarization plane rotating section 1066 e, polarizers1066 f, 1066 g, branches light from the polarized wave controller 566,inputs one branched light to the measuring object 1, outputsinterference light by multiplexing other branched light (referencelight) with output light (signal light) outputted from the measuringobject 1, inclines an optical axis of output light and an optical axisof reference light to provide a predetermined angle to an optical axisangle formed by the two optical axes to be multiplexed to form spatialinterference fringes.

Respective HM 1066 a, the mirror 1066 b, PBS 1066 d, the polarizationplane rotating section 1066 e, the polarizers 1066 f, 1066 g are similarto HM 666 a, the mirror 666 b, PBS 666 d, the polarization planerotating section 666 e, the polarizers 666 f, 666 g and an explanationthereof will be omitted.

The mirror 1066 c is installed such that although the mirror 1066 creflects reference light reflected by the mirror 1066 b and having apolarization plane inclined by the polarization plane rotating section1066 e, an optical axis of reference light after having been multiplexedand branched by PBS 1066 d and an optical axis of signal light are notin parallel with each other and signal light and reference light aremultiplexed while being shifted from each other by a small angle.Thereby, the interference section 1066 generates interference fringes inan optical intensity distribution in a beam face of interference light.

The photodiode arrays 1166, 1266 each includes 4 pieces of photodiodes.An explanation will be given in details in reference to FIG. 11. Both ofthe photodiode arrays 1166, 1266 are constructed by the sameconfiguration and therefore, an explanation will be given byillustrating the photodiode array 1166.

The photodiode arrays 1166, 1266 each includes photodiodes P(1) throughP(4). Each of the photodiodes P(1) through P(4) receives light byequally dividing one spatial period of interference fringes formed bythe interference section 1066 by four. Naturally, the photodiodes P(1)through P(4) are aligned by being shifted along a direction of forminginterference fringes. In other words, the photodiodes P(1) through P(4)are arranged by shifting phases thereof by 90° at the period of theinterference fringes.

Here, an optical intensity distribution 1006 of the interference lightsfrom PBS 1066 d in FIG. 11 schematically shows an optical intensity ofthe interference fringes formed on light receiving faces of thephotodiodes P(1) through P(4).

The optical intensity constitutes such interference fringes because asdescribed above, the optical intensity distribution 1006 shown in FIG.11 is generated in the beam face of interference light by multiplexingreflected light (reference light) from the mirror 1066 c and outputlight (signal light) of the measuring object 1 by PBS 1066 d byinclining wave faces thereof.

Further, in FIG. 11, a first, a second, a third, a fourth areconstituted from the photodiode P(1) on the left side. Further, lightnon-receiving sections among the photodiodes P(1) through P(4) may bereduced such that light receiving sections of the photodiodes P(1)through P(4) are provided with a width constituted by equally dividingone spatial period of the interference fringes by four.

Further, the period of the interference fringes differs by a wavelengthof measured light and therefore, for example, in a center wavelength ina wavelength measuring range, a width of a total of 4 pieces of thephotodiodes P(1) through P(4) and the period of the interference fringesmay coincide with each other.

Specifically, when an angle of inclining wave faces of signal light andreference light is increased, an interval between the interferencefringes is narrowed and when the inclined angle is reduced conversely,the interval between the interference fringes is widened. Further, whenthe angle of inclining the wave faces is finally nullified (parallel), auniform optical intensity is achieved. Therefore, the width is made tocoincide with the period of the interference fringes by a desiredwavelength by adjusting to incline the mirror 1066 c in consideration ofa light receiving width of the photodiodes P(1) through P(4), theinterval between the interference fringes and the like.

Further, the interference fringes are moved in a transverse direction(direction of aligning the photodiodes P(1) through P(4)) by a change ina phase difference of signal light and reference light, that is, by awavelength of laser light.

The interference signal converting sections 1366, 1466 each includes 2pieces of subtracting circuits, generates a first, a second interferencesignal phases of which are shifted from each other from respectiveoutputs of the photodiode arrays 1166, 1266 to be outputted to thecalculating section 966. Both of the interference signal convertingsections 1366, 1466 are constructed by the same configuration andtherefore, an explanation will be given by illustrating the interferencesignal converting section 1366.

The interference signal converting sections 1366, 1466 each includessubtracting circuits A1, A2. The subtracting circuit A1 outputs a resultof subtracting an output of the third photodiode P(3) from an output ofthe first photodiode P(1) to the calculating section 966 as the firstinterference signal. The subtracting circuit A2 outputs a result ofsubtracting an output of the fourth photodiode P(4) from an output ofthe second photodiode P(2) to the calculating section 966 as the secondinterference signal. Therefore, phases of the first interference signaland the second interference signal are shifted from each other and thephases are shifted from each other by 90° by a predetermined wavelength(for example, center wavelength in measured wavelength range.

Operation of the apparatus will be explained.

Wavelength sweep is carried out twice in a predetermined wavelengthrange in order to input respective p polarized light and s polarizedlight to the measuring object 1 similar to the apparatus shown in FIG.17. First, first wavelength sweep will be explained.

First, at first wavelength sweep, similar to the apparatus shown in FIG.17, laser light (p polarized light) of parallel light outputted from thewavelength variable light source 2 and passing through the optical fiber366, the lens 466, the polarized wave controller 566 is inputted to theinterference section 1066.

Further, HM 1066 a branches light from the polarized wave controller566, outputs one thereof to the measuring object 1 as signal light andoutputs other thereof to the mirror 1066 b as reference light. Further,the polarization plane rotating section 1066 e inclines a polarizationplane of reflected light from the mirror 1066 b by 45° relative to theoptical axis PBS 1066 d to be outputted to the mirror 1066 c such thatoptical power is uniformly branched at PBS 1066 d at a rear stage.Further, an optical axis of reflected light to the mirror 1066 c and anoptical axis of output light of the measuring object 1 may be made to bein parallel with each other.

Further, an optical axis of reflected light by the mirror 1066 c is notorthogonal to the optical axis of output light from the measuring object1 but shifted therefrom slightly to be inputted to PBS 1066 d. Further,PBS 1066 d multiplexes output light (emitted s polarized light, emittedp polarized light in correspondence with inputted p polarized light)from the measuring object 1 and reference light by way of the mirrors1066 b, 1066 c to be branched in two of light (p polarized light, spolarized light) polarization planes of which are orthogonal to eachother.

Further, according to multiplexed light outputted from PBS 1066 d, anoptical axis angle formed by the optical axis of signal light and theoptical axis of reference light is provided with a small angle. Thereby,the spatial interference fringes are formed on light receiving faces ofthe photodiode arrays 1166, 1266. Naturally, output light from themeasuring object 1 is signal light. Further, both of signal light andreference light to be multiplexed may be constituted by parallel light.

Further, polarization planes of light (signal light and reference light)multiplexed and branched by PBS 1066 d are orthogonal to each other, thepolarization planes are inclined by the polarizers 1066 f, 1066 g to beinterfered with each other to be received by the photodiode arrays 1166,1266.

Further, the photodiode array 1166 is inputted with interference lightof signal light operated by T₂₂ of Jones matrix and reference light.Further, the photodiode array 1266 is inputted with interference lightof signal light operated by T₁₂ of Jones matrix and reference light.

The respective photodiodes P(1) through P(4) of the photodiode arrays1166, 1266 receive multiplexed interference light from PBS 1066 d andoutput electric signals in accordance with optical power of receivedinterference light to the interference signal converting sections 1366,1466.

Further, the subtracting circuits A1 of the interference signalconverting sections 1366, 1466 calculates (output of first photodiodeP(1))−(output of third photodiode P(3)) and outputs a result ofsubtraction to the calculating section 966 as first interferencesignals.

Further, the subtracting circuits A2 of the interference signalconverting sections 1366, 1466 calculate (output of second photodiodeP(2))−(output of fourth photodiode P(4)) and outputs a result ofsubtraction to the calculating section 966 as second interferencesignals. Naturally, also amounts of offset of both of the first and thesecond interference signals are removed.

The calculating section 966 calculates a moving direction and an movingamount the interference fringes from the first and the secondinterference signals phases of which are shifted from each other by 90°.That is, because the moving direction and the moving amount correspondto an increase or a decrease of the phase difference of multiplexedlight.

Successively, second wavelength sweep is carried out and a point of thesecond wavelength sweep which differs from the first wavelength sweepresides in that the polarized wave controller 566 converts laser lightinto s polarized light, that the photodiode array 1166 is inputted withinterference light of signal light operated by T₂₁ of Jones matrix andreference light, that the photodiode array 1266 is inputted withinterference light signal light operated by T₁₁ of Jones matrix andreference light, and the other operation is similar to the firstwavelength sweep and therefore, an explanation thereof will be omitted.

Further, the calculating section 966 calculates respective elements ofJones matrix from phases and amplitudes of the interference signalsbased on respective p polarized light, s polarized light and calculatesan optical characteristic of the measuring object 1 from calculatedJones matrix.

In this way, the interference section 1066 forms the spatialinterference fringes by shifting the optical axis of signal light andthe optical axis of the reference light from each other to be interferedwith each other. Further, interference light is received by four piecesof the photodiodes P(1) through P(4) phases of which are shifted fromeach other by 90° relative to the period of the interference fringes.Further, the interference signal converting sections 1366, 1466 generatethe first and the second interference signals phases of which areshifted from each other by 90° from the signal of the photodiode array1166. The calculating section 966 calculates the moving direction andthe moving amount the interference fringes from the first and the secondinterference signals and therefore, an amount of a phase including anincrease or a decrease in the phase difference of multiplexed light iscalculated. Thereby, an increase or a decrease in the phase differenceof light (signal light and reference light) to be multiplexed can easilybe determined. Therefore, the optical path length of the measuringobject is not limited.

Seventh Embodiment

FIG. 12 is a configuration diagram showing a seventh embodiment of theinvention. Here, sections the same as those of FIG. 10, FIG. 11 areattached with the same notations and an explanation thereof will beomitted. In FIG. 12, pluralities of pieces of the photodiode arrays1166, 1266 are provided along a direction of forming the interferencefringes from the interference section 1066. Further, in FIG. 12, onlythe photodiode array 1166 is illustrated and an explanation will begiven of the side of the photodiode array 1166.

Outputs of first ones of the photodiodes P(1) and outputs of third onesof the photodiodes P(3) of the respective photodiodes arrays 1166 areinputted to the subtracting circuit A1 to be subjected to subtractionand outputted as the first interference signals.

Further, outputs of second ones of the photodiodes P(2) and outputs offourth ones of the photodiodes P(4) of the respective photodiodes arrays1166 are inputted to the subtracting circuit A2 to be subjected tosubtraction and outputted as the second interference signals. That is,the photodiodes P(1) through P(4) are wired at every 4 pieces thereof.

In this way, pluralities of pieces of respective the photodiode arrays1166, 1266 are provided along a direction of aligning the interferencefringes, and the interference signal converting sections 1366, 1466generate the interference signals from the outputs of the pluralities ofphotodiode arrays 1166, 1266. Thereby, even when there is anonuniformity (random noise) at a section or a total of the interferencefringes, the interference signal which is less influenced by thenonuniformity can be provided by averaging.

Eighth Embodiment

Although according to the optical characteristic measuring apparatusshown in FIG. 10, FIG. 12, at a predetermined wavelength, the spatialperiod of the interference fringes and the period of the photodiodearrays 1166, 1266 coincide with each other, the more remote from thepredetermined wavelength, the more shifts the period.

FIG. 13 is a configuration diagram showing an eighth embodiment of theinvention, which can calculate a moving amount the interference fringes,that is, the phase difference of multiplexed light with high accuracy.Here, sections the same as those of FIG. 10 are attached with the samenotations and an explanation thereof will be omitted. In FIG. 13, thecalculating section 966 is provided with correcting section 966 a.

The correcting section 966 a is inputted with the wavelength of laserlight which is being outputted from the wavelength variable light source2 and corrects an error in the moving amount the interference fringes bythe shift between the spatial period of the interference fringes and theperiod of the photodiodes P(1) through P(4) of the photodiode arrays1166, 1266.

Operation of the apparatus will be explained.

The correcting section 966 a is inputted with the wavelength of laserlight which is being outputted (for example, may be with a roughaccuracy not by [pm] unit but by [nm] unit) from the wavelength variablelight source 2. Further, the correcting section 966 a calculates theshift between the spatial period of the interference fringes and theperiod of the photodiodes P(1) through P(4) of the photodiode arrays1166, 1266 from the wavelength of the rough accuracy.

That is, the interference fringes are moved by depending on thewavelength of the laser light outputted from the wavelength variablelight source 2. However, the period of the interference fringes ischanged by the wavelength. Therefore, the correcting section 966 acalculates the moving amount the interference fringes relative to anamount of a change in the wavelength by calculation in consideration ofa change in the period. Or, the shift between the periods depending onthe wavelength is previously measured or calculated to be stored to amemory (not illustrated). Further, the correcting section 966 acalculates the moving amount by the phase difference of multiplexedlight by removing an influence of a change in the period of theinterference fringes produced by the wavelength sweep.

That is, the shift in the period is uniquely determined by thewavelength and therefore, the correcting section 966 a calculates theshift in the period by calculation or data stored to the memory.

Further, the correcting section 966 a corrects an amount of error of themoving amount produced by the shift between the periods. Further, thecalculating section 966 calculates Jones matrix of the measuring object1 from phases and amplitudes of the interference signals based on thecorrected moving amount. Other operation is similar that of theapparatus shown in FIG. 10 and therefore, an explanation thereof will beomitted.

In this way, the correcting section 966 a corrects the error in themoving amount produced by the shift between the periods from thewavelength with rough accuracy and therefore, an increase or a decreasein the phase difference can accurately be calculated.

Further, the invention is not limited to the sixth to the eighthembodiments but may be as shown below.

Although in the apparatus shown in FIG. 10 and FIG. 13, there is shown aconfiguration of providing an interferometer of Mach-Zender type of theinterference section 10660, any two light flux interferometer may beused, for example, a Michelson type interferometer will do. In sum, anyinterferometer will do so far as the interferometer generatesinterference fringes in a linear shape by multiplexing signal light andreference light in a state of inclining wave faces thereof.

Although in the apparatus shown in FIG. 10 and FIG. 13, there is shown aconfiguration of using HM 1066 a, any element will do so far as elementbranches light without depending on a polarized state, for example, anon-polarization beam splitter, an optical fiber coupler or the likewill do.

Although in the apparatus shown in FIG. 10, FIG. 13, there is shown aconfiguration of inclining the mirror 1066 c, the optical axis ofreference light and the optical axis of signal light multiplexed by PBS1066 d may be adjusted to a slightly inclined state and as a method ofproducing inclination of the two optical axes, the mirrors 1066 b, 1066c may be inclined, or HM 1066 a, PBS 1066 d may be inclined.

Although in the apparatus shown in FIG. 10, FIG. 13, there is shown aconfiguration of providing the polarized wave controller 566 between thelens 466 and the interference section 1066, the polarized wavecontroller 566 may be provided at inside of the wavelength variablelight source 2.

Although in the apparatus shown in FIG. 10, FIG. 13, there is shown aconfiguration of carrying out twice wavelength sweep (for outputting ppolarized at first time and outputting s polarized light at second time)by the wavelength variable light source 2, the respective elements ofJones matrix may be calculated by one time wavelength sweep.

For example, the light source section outputs first, second input lightfrequencies of which differ from each other and polarized states ofwhich are orthogonal to each other to the interference section 1066.Here, the first input light is constituted by p polarized light(frequency f1(t)), and the second input light is constituted by spolarized light (frequency f2(t), however, f1(t)≠f2(t)). Further, thelight source section 2 carries wavelength sweep (frequency sweep) bymaking a frequency difference (|f1(t)−f2(t)|) substantially constant tobe outputted to the interference section 1066. On the other hand, insignals from the photodiode arrays 1166, 1266, there are presentinterference signals of emitted p polarized light, emitted s polarizedlight and reference light respectively in correspondence with incident ppolarized light, incident s polarized light. However, frequencies ofincident p polarized light, incident s polarized light differ from eachother and therefore, a difference in beat frequencies of theinterference signals produced by the frequency difference may befiltered to separate the interference signal of s polarized light andreference light, the interference signal of p polarized light andreference light.

Further, two pieces of wavelength variable light sources may be preparedfor the light source section for outputting the first, the second inputlight. Further, the frequency difference may be produced by branchinglight of one piece of the wavelength variable light source in two anddelaying one light, or the frequency difference may be produced byshifting the frequency of one light (for example, by usingacousto-optical modulator).

Although in the apparatus shown in FIG. 11, there is shown aconfiguration in which the photodiode arrays 1166, 1266 each includes 4pieces of the photodiodes P(1) through P(4), any number of piecesthereof will do, for example, the photodiode arrays 1166, 1266 each mayinclude at least (4×n) pieces of the photodiodes, and respective thephotodiodes may receive light by equally dividing one spatial period ofthe interference fringes by four (that is, by installing the respectiveby shifting the phases by 90° relative to the period of the interferencefringes).

Further, the interference signal converting sections 1366, 1466 each mayoutput a result of subtracting an output of the (4×(i−1)+3)-thphotodiode from an output of the (4×(i−1)+1)-th photodiode as the firstinterference signal and may output a result of subtracting an output ofthe (4×(i−1)+4)-th photodiode from an output of the (4×(i−1)+2)-thphotodiode as the second interference signal. Incidentally, notations n,i designate natural numbers.

That is, the interference signal converting sections 1366, 1466 eachoutputs a result of subtracting the output of (3, 7, 11, . . . )-thphotodiode from the output of (1, 5, 9, . . . )-th photodiode as thefirst interference signals and outputs a result of subtracting theoutput of (4, 8, 12, . . . )-th photodiode from the output of (2, 6, 10,. . . )-th photodiode as the second interference signals.

In this way, the photodiode arrays 1166, 1266 measure a plurality ofperiods of the interference fringes, and the interference signalconverting sections 1366, 1466 generate the interference signals fromthe outputs of the photodiode arrays 1166, 1266. Thereby, even when anonuniformity (random noise) is present at a section or a total of theinterference fringes, the interference signal which is less influencedby the nonuniformity can be provided by averaging.

Although in the apparatus shown in FIG. 12, there is shown aconfiguration of aligning the photodiode array 1166 without a gaptherebetween, a gap may be provided between the photodiode arrays 11.

Although in the apparatus shown in FIG. 13, there is a shown aconfiguration of inputting the wavelength which is being outputted fromthe wavelength variable light source 2 to the calculating section 966,when the correcting section 966 a is provided with start wavelength,sweep speed in carrying out wavelength sweep, an error by a shift in theperiod may be corrected by start wavelength, sweep speed.

It will be apparent to those skilled in the art that variousmodifications and variations can be made to the described preferredembodiments of the present invention without departing from the spiritor scope of the invention. Thus, it is intended that the presentinvention cover all modifications and variations of this inventionconsistent with the scope of the appended claims and their equivalents.

1. An optical characteristic measuring apparatus for measuring an optical characteristic of a measuring object, the optical characteristic measuring apparatus comprising: an interference section which branches light from alight source section, inputs one branched light to the measuring object, and makes other branched light interfere with output light being outputted from the measuring object so as to form interference fringes by multiplexing the output light and the other branched light while inclining an optical axis of the output light and an optical axis of the other branched light, wherein a moving direction and a moving amount of the interference fringes are measured.
 2. The optical characteristic measuring apparatus according to claim 1, further comprising: at least one photodiode array which includes a plurality of photodiodes and receives an interference light from the interference section, the photodiodes being arranged to be shifted along a direction in which the interference fringes are formed; and an interference signal converting section which generates a plurality of interference signals from an output of the photodiode array, phases of the plurality of interference signals being shifted from each other.
 3. The optical characteristic measuring apparatus according to claim 2, wherein the plurality of photodiodes includes at least four photodiodes, and the respective photodiodes receive light by equally dividing one spatial period of the interference fringes by four.
 4. The optical characteristic measuring apparatus according to claim 3, wherein the interference signal converting section outputs a subtraction result of an output of a third photodiode of the photodiodes and an output of a first photodiode of the photodiodes as a first interference signal, and the interference signal converting section outputs a subtraction result of an output of a fourth photodiode of the photodiodes and an output of a second photodiode of the photodiode as a second interference signal.
 5. The optical characteristic measuring apparatus according to claim 2, wherein said at least one photodiode array includes a plurality of photodiode arrays which are arranged along a direction in which the interference fringes are formed.
 6. The optical characteristic measuring apparatus according to claim 2, wherein the plurality of photodiodes includes at least (4×n) photodiodes, the respective photodiodes receive light by equally dividing one spatial period of the interference fringes by four, the interference signal converting section outputs a subtraction result of an output of (4×(i−1)+3)-th photodiode of the photodiodes and an output of (4×(i−1)+1)-th photodiode of the photodiodes as a first interference signal, and the interference signal converting section outputs a subtraction result of an output of (4×(i−1)+4)-th photodiode of the photodiodes and an output of (4×(i−1)+2)-th photodiode of the photodiodes as a second interference signal, where notations n, i designate natural numbers.
 7. The optical characteristic measuring apparatus according to claim 2, further comprising: a correcting section for correcting a difference between a spatial period of the interference fringes and a period of the photodiodes of the photodiode array. 